The 2023 National Electric Code kicks off its interconnection discussion with clear examples of AC and DC interconnection examples. This elicits a popular debate over whether AC or DC coupling is better, as well as a more recent debate as to whether listed DC coupled systems must co-list the AC inverter. The DC interconnected example reflects hybrid inverters, even detailing the possibility of having advanced grid controls built directly into the inverter. But the graphic implies the UL9540-2020 Edition definition of a DC ESS (which includes the inverter) as opposed to the UL9540 2016 and 2023 Editions which may list battery systems without any inverter co-listing. That is a subject better explored in a conversation about UL9540, but it’s worth mentioning that DC ESS with inverter agnostic compatibility is once again allowed, after being previously disallowed in the prior code cycle. So there’s still a lot going with the definitions of interconnection. “Stand alone mode” is the term for off-grid. “Grid Interactive mode” is the term for grid-tied. And an inverter which can do both is a “multi-modal” inverter.

NEC 705’s AC interconnection example is similar to traditional solar battery systems, where solar is on its own inverter, and the batteries are on their own inverter, like early models of the Tesla Powerwall. Essentially, AC-coupled is when you have a lot of inverters wired in parallel to each other on the AC side. Often, that might consist of “multi-modal” battery inverters which and “grid interactive” solar inverters which follow along whatever power source is forming the grid.

The debate as to which architecture is better is overblown. Both are adequate for grid-tie backup. There are slight differences, which become more important for advanced design such as off-grid or very large applications. It’s good ot know AC versus DC coupling advantages and disadvantages. AC coupling might be less elegant and controllable, but for commercial rooftops, it is very practical due to availability of built-to-purpose inverters like the inexpensive SMA Sunny Tripower. My design preference is for solar DC coupling with hybrid inverters, but hybrid inverters are best located near either the batteries or points of interconnection. If the solar array is far away from that point, it can make since to combine all the solar circuitry together on the roof with a dedicated solar inverter and then AC-couple to the hybrid inverter GEN port, such as found on the Sol-Ark 30K-208V or 60K-480V hybrid inverters.

Now, if that were a solar carport, where the interconnection, solar array, and batteries are all located in the parking lot, DC coupling is ideal. If anything, it keeps the GEN port available on hybrid inverters for use with other items, such as convenient EV charging interconnection (as makes sense in a solar carport). And for residential application, I prefer DC coupling simply because it is less complicated to reduce the number of power sources within a system, which as an increasingly important consideration if you want to incorporate other sources like gas generators or multi-modal EV charging. But if the solar array already exists, or if the installer is wed to using microinverters on the rooftop, then AC-coupling gets the job done too.

DC coupling has a variety of advantages, but the most basic is that it runs at higher voltage than AC building electricity. The higher the voltage, the lower the amperage for the same amount of power. And the diameter of that copper power cable has more to do with amperage than voltage. So for example where the solar array is far away from the inverter, such as a ground mount or far array roof mount, it makes sense to stay with DC coupling. Also, pragmatically, a hybrid inverter has DC-coupled solar ports. It’s a shame not to use them. There’s nothing more simple than taking the DC strings directly from the solar array and landing them directly onto the inverter, even with a long distance in between. in and out of the home and under… Through some roads, and the rest being direct burial cable.

The fact of the matter is that any site with multiple inverters uses AC-coupling. There is a debate as to whether or not it is better to use one big inverter verses a few smaller inverters, if not dozens or hundreds. From a cost and simplicity perspective, one big thing usually wins the debate compared to hundreds of small things. This is why I like the Sol-Ark whole home backup hybrid inverter specifically. At 15kW AC output, it is the largest residential inverter on the market in terms of AC power output, which results in the most simple system architecture. It also manages the grid interconnection internally, meaning no additional equipment is necessary to have full access to grid power, while providing power to all home circuits during an outage. Sol-Arks competitors offer inverters between 5kW and 12kW, meaning multiple ESS have to be installed, as well as an energy management panel, to have similar or better functionality. Line item by line item, an individual part of a smaller AC ESS might be less expensive than a SolArk. But a the bird’s eye view of the project will show that designing and installing around bigger building blocks will always be cost-effective, and large form factor hybrid inverter architecture simplifies things further by combining battery, grid, and interconnection scope.

The term “whole home backup” really comes from the generator industry. “Whole Home” generators have been sized between 20kW and 24kW traditionally. Now remember that a typical residential electric service is 200 amps at 240 volts, in other words, 48 kilowatts. So a “whole home” backup generator is not actually capable of running every circuit in the house simultaneously, and neither are typical whole home backup ESS.

The question is, where do you interconnect a home hybrid inverter? Do you do interconnect like traditional solar installations, on the load-side breaker at the bottom of the service panel? Or do you intercept the conductors between the meter and the main breaker? For “whole home” hybrid inverters, the common path is to do a supply-side connection, between the utility meter and the main service panel. This is a very meaty connection. Fundamentally, the conductors between the meter and the main service panel are rated for the service connection. If the home has 200 amp electric service, the service conductors are rated for at least rated for 200 amps. A 200A system could be interconnected and feed the service. The home doesn’t care if the 200A come from the grid or home.

Regarding 200A connections, evaluate the inverter terminations closely. It’s common for smaller hybrid inverters to include breaker switches for grid, load, and generator connections, but the larger the inverter model, the less switches are included. This is because the breakers themselves get bigger. A 200A breaker will not fit on the load side of a service panel, and for the same reason, there are limitations to what can be fit into inverter wiring boxes. Furthermore, NEC requires a readily accessible AC ESS outside the home, and it gets expensive to purchase redundant high amperage disconnects. So the Sol-Ark 15K only has a load and battery disconnects. Likewise the 30K and 60K commercial models do not include any breaker disconnects. Whereas the Sol-Ark 12K has breakers for the load, grid, and generator, which are sized similar to breakers found on the load-side of a residential service panel.

It’s obvious that a breaker can be used to interconnect an device to a service panel. But how does a supply-side interconnection work? Supply side taps which intercept the conductors between the meter and the main service panel typically require a site power down. The electric meter is pulled, and the taps are installed. the site, you have to pull the meter, and then taps are installed, providing an additional interconnection point. The tap has an amperage rating as well. But from the electric service panel point of view, it doesn’t matter how large the source is, the panel will only be fed 200A from the main breaker, because if more power is drawn, it will trip the main breaker. So supply-side connections can feed feed as much as 200 amps through the meter into the panel. This is precisely what the “grid pass through” capability of a whole home hybrid inverter does. Although the Sol-Ark 15K can only pass through 180A continuously, most homes don’t get anywhere near that. If they do, they’re already upgrading to 400 amp services. Sol-Ark increases its pass-through amperage cumulatively, such that a minimum “whole home backup” system design of 400A service would require at least two Sol-Ark 15Ks, perhaps double lugged at the external ESS disconnect, and then landed onto a 400A Main Lug Only panel, making effective use of the inverter harder.

Proper load side interconnections used to be a very important subject in solar design, with the famous 120% Rule, which in short, allows up to a 40A source breaker on the load side of a 200 amp service panel. Those rules have gotten much more flexible, as we’ll learn in a few slides. Despite being designed for high amperage, supply side connections, whole home hybrid inverters can be interconnected to the load side of the service panel in a few cases. It’s less of a question as to what is possible, and more of a question of “why would you?”, but there are a few good answers. At the very least, some service panels, like certain meter-based combo panels, force the designer’s hand and require a load-side connection.

One method to circumvent the 120% rule is to list the inverter as a power control system, capable of monitoring the busbar and adjusting the inverter output to ensure the busbar is not overloaded. This UL1741CRD listing allows any sized of listed-PCS inverter to be connected to the load side of a service panel. The historic concern was that the grid could supply 200A of power to the busbar and the ESS could supply, say, 100A of power to the busbar, and that the busbar on a 200A service panel is rated for 200A. Count up the breakers on your electric service panel and they likely add up to around that much, and if all those devices were used simultaneously, it could cause the service panel to overheat, creating a fire hazard. But the UL 1741CRD listing, which is a special PCS listing beyond standard UL1741, which indicates additional monitoring and control capability, will restrict the power supplied to the busbar to within its limits, in this case, 200A. Both the SolArk 12K and the SolArk 15K are PCS rated, and so can be load-side connected to the busbar.

The term PCS is slightly problematic. If the term PCS sounds familiar, PCS inverters are already used in industry to indicate very large commercial battery inverters, which have slightly different architecture than residential inverters. While a residential hybrid inverter may be a listed PCS, it is not quite the same PCS as how the term “power control system” is used in commercial energy storage. Also, there are products which can monitor and control busbar voltage that are not inverter, such as Span and Lumin smart panels. This National Electric Code busbar interconnection exemption to the 120% rule has now relabeled PCS as EMS, or “energy management system”, and so the UL1741CRD listing confers PCS status to the inverter, which is a type of EMS, for the purposes of exempting from the 120% rule.

Despite losing whole home backup capability, here are some values in keeping the interconnection on the load-side, despite it being less common. While losing whole home backup capability, connecting to the load side of the service panel means the electricians don’t need the power company to visit site to power down the electric service, so the installation is faster. Secondly, there might be an impossibility of a supply-side connection, such as certain meter-based all-in-one panels. Third, an existing 200A service might be overloaded to the point a 400A service upgrade is required to add new devices like EV chargers. Instead, adding a battery inverter to power the device prevents overloading the panel, saving the cost of the upgrade.

The Sol-Ark 12K makes for a very easy load-side interconnection, because it includes breakers for the load, grid, and gen ports. Why might a Sol-Ark 15K be used instead of a Sol-Ark 12K? To start out with, the 12K is unfortunately named, only having 9kW of AC output capacity. At the time of its release, it seemed innovative to add an additional 3kW of DC side charging power, such that under the right circumstance, it can process the same amount of power as a 12 kilowatt inverter. But ultimately, AC power is more important than DC power, and the inverter is only 9kW AC. So the 15KW, even with providing external disconnects, is often cost-effective still, because of the additional 6kW of AC output capability. That’s the difference between running large HVAC systems in parallel with other loads, or the ability to do laundry and and cook dinner at the same time.

At any rate, when performing a load-side connections, the AHJ may require use of Sol-Ark’s user lockout feature to restrict access to the inverter settings, in the instances where the inverter capability exceeds the interconnected power run between the inverter unit and the main service panel. While this run is also protect by in-line overcurrent protection at the breaker, its not a good practice to rely upon safety systems to govern normal system operation. Locking the user out of the settings changes, and that’s done with SolArk by two ways. One, you can lock them out of the LCD screen locally as the last part of system commissioning. After checking the locking setting on the LCD screen, it will require a support call to Sol-Ark to get the unlock code. The second step in locking out the user from settings adjustments is to designate them as a viewer rather than manager within the user profiles of the online monitoring system.

Now that supply and load side connections have been discussed, it’s time to point out some additional flexibility found on most hybrid inverters, which is an additional GEN Port. The key takeaway here is this the GEN port is another tap point of connection that can be used to land a generator onto the system, but it can also be used for other devices, such as AC coupled solar, an additional electrical sub-panel like an outdoor panel, or an electric vehicle charger. Regardless of how many inverters are installed, this GEN port can only be programmed for one functionality.

So that Gen Port is incredibly flexible. On the 15K model, it is large enough for 24kW generators, although output will be throttled down to 19.2W. The Gen Port is rated to 100 amps max, but is software constrained to 80 amp continuous. On the 12K model, the GEN port is half the size, and so can accommodate smaller portable generators or EV chargers.

Now, the Gen Port is bidirectional, and one way to think about the Gen Port is that it is purely an additional tap that you can put high amperage devices onto, that is a sheddable relay. And so it’s intended purpose is to disconnect if the total system is overloaded, and also act as a load shed for when the batteries are at low states of charge.

If you can think of no other use for the GEN port, then don’t forget the smart load function, which can provide battery state-of-charge load control to any device or subpanel. In principle, I’m skeptical of load control. Imagine managing your electricity by standing in front of a service panel, periodically turning breakers on and off. It does not make sense much sense for most home devices, which could otherwise be managed through manually such as recognizing a power outage and then adjusting the thermostat setting, remembering not to do laundry and cook at the same time. But there are some energy management advantages of moving all miscellaneous loads to a subpanel which sheds during low battery states of charge. This technique can significantly extend the backup capability of a home, and because it only requires a generic subpanel, it is much less costly than the installation of a traditional smart panel.

At the very least, using the GEN port for an electric tank water heater is almost like gaining a free hot water battery, because it can heat hot water with solar production. When the smart load state-of-charge controller is combined with the inverter time-of-use controller, this functions as a load shed, only enabling hot water during the day, when solar is likely available. And both inverters and tank water heaters are commonly located in shared spaces like garages or utility closets, so this is a relatively easy scope addition to a residential project.

What I like most about hybrid inverter GEN ports is it provides a flexible architecture. If something was missed in the design process, whether it be an existing solar array, or the need for upgraded electric service such as when adding an EV charger, or both indoor and outdoor service panel, it provides a solution for the challenge at hand. So the Sol-Ark 15K deserves credit for being flexible enough to accommodate an incredibly wide range of jobsites and applications, helping installers resolve otherwise costly design mistakes or additions, as well as simplify inventory management.

What about a vehicle-to-load export power from EV? Can the EV output look like a generator input to the Sol-Ark GEN input, in addition to charging the vehicle? The short answer is yes, but no specific multi-directional EV charger settings have been programmed into the inverter. The ability to add dozens of kwh of storage to the home during a grid outage is revolutionary to ESS design.

But while it’s relatively easy to charge your electric vehicle during a grid outage, being able to discharge in concert with a manufacturer agnostic platform is still in its infancy. If the ability to discharge the electric vehicle charger is desired, I recommend thinking of the EV charger as a battery inverter. Is it capable of discharging to the grid? Is it capable of forming its own grid? If capable of forming its own grid, is it programmed to synchronize to the ESS instead? What is the relative side of the charger compared to the load? Uni-directional EV charging is very simple, but bidirectional EV charging still requires expertise. Keep thinking creatively about incorporating EV charging into solar and battery design. In the future, you might even incorporate batteries and solar into the EV charger system, rather than the other way around. For example, the Sol-Ark 8k-1p is not capable of whole home back on its own, but can charge an EV. Together, any V2H whole home backup system could act as the grid to the 8k-1p, providing a way to backup the home and charge the car, erstwhile prioritizing lowest possible project cost.

One important item to note, is that when installing with multiple inverters, it’s very easy to imagine the GEN port on each inverter managing different features, like using one port for a generator connection and another for a smart load panel. But unfortunately this is a case of imagination exceeding reality. Balanced against all-expansiveness is reliability – there are communication and monitoring errors that become more complicated with inverters wire in parallel as part of a single system, and Sol-Ark errs on the side of reliable, restricting the GEN port to a single purpose. If multiple GEN port features are desired, then some of those devices will need to be interconnected on the LOAD side, such as a Span or Lumin smart panel.

Let’s talk about transfer switch capabilities and options. When a whole home ESS fails, the concern becomes whether or not the building has any power source at all. Any ESS architecture, hybrid or otherwise, has some microgrid interconnect device between the home and the grid, so what happens if that device fails? Keep in mind this is a small fraction of possible inverter failures – in other words, there are many cases where an inverter can fail and keep the building powered with grid or generator power. Nonetheless, its good, but not mandatory, to have a backup plan for when the backup plan is in servicing. There’s different ways to do this, with a variety of cost options.

The most expensive and professional option is a manual bypass switch to reroute grid power to the home. While expensive, keep in mind an external ESS grid disconnect is already a project requirement. Landing additional, fused conductors onto the bypass meets all disconnect requirements and provides full bypass power to the building. For example, crimp-on fuses are useful here, as most of these manual throw switches are not fused throw switches. There is also wisdom in adding a second inverter for redundancy, if the budget allows and power demands are justifiable.

That said, for projects on a budget, a smaller option may be sufficient for service power, to power essential loads. This less expensive option is not intended for whole building temporary power, but is sufficient to provide a small amount of power when the system is being serviced. This solution is a generator interlock switch. And be careful implementing these as some jurisdictions require the service panel be intended for a generator interock switch with pre-drilled holes for mounting, because as you can see from this photo, otherwise the service panel must be modified to accomodate the security plate added to the panel. The concern being that drilling holes in service panel can violate its UL listing. It is strongly recommended when retrofitting to confirm service panel brand and model compatibility. Eaton panels are coming with pre-punched zones for Eaton-supplied interlocks.

What this Generator Interlock Switch does is lock the main breaker coming from the ESS out of the system, providing a a grid bypass the size of whatever load-side breaker is installed at the top of the busbar. Assuming the ESS has an external disconnect, double lug taps could be installed to connect both the 200 amp ESS grid input, as well as the smaller interlock redirect, let’s call it 50A. At the very least, that would allow the homeowner to keep all their refrigerator, kitchen, garage, well pump, and mini-split running while the inverter is being replaced. At the very least that can keep the system owners in good spirits. The generator interlock solution isn’t for everyone. Many installers will find it too cheap. Many installers, the customers would just prefer a 200 amp throw switch, or something even more automatic than that. But knowing the option exists can be a way to differentiate yourself from being the lowest bid installer.

We’ve covered whole home backup in great detail, so let’s talking about some not-whole-home interconnection options, which might be very lightweight or middle-of-the-road regarding budget and capabilities. Sol-Ark has a Lite option, the 8K-1P is that is pure 240V AC, rather than other Sol-Ark inverters producing 120V/240V split phase service. Because of this, the Sol-Ark Lite can’t be used for home backup power. Technically it could still backup 240V circuits like a dryer, water heater, or EV charger. Which going back to the EV charger discussion earlier, could provide some interesting vehicle-to-home compatibility. The great thing about the 8K-1P is its price. The cost of that 120/240V transformer has to come from somewhere. So the Sol-Ark Lite option is the cheapest entry point to Sol-Ark ESS ownership.

I’m super excited about this inverter, because I rent, so I can’t put solar on my roof. But I can interconnect a battery system to my service panel and use it to lower my time-of-use electric rate.

Another great use of the Sol-Ark Lite is to expand existing Sol-Ark systems. It’s not a good idea to parallel batteries which have different usage amounts, not to mention models or chemistries, and sometimes you want to expand system power or storage capability without paying for all of that expansion upfront. In those cases, the 240V inverter AC-coupled into the system is run in demand-management mode, only providing power when the system goes above a certain power threshold. This can significantly extend the life of an older battery bank that can only contribute power at lower amperage rates. The weaknesses to this technique is online monitoring, as the independent systems would essentially be monitored independently. But whether its small time-of-us metering where backup isn’t important, or to add additional power or energy to an existing system, the 8K-1p is definitely worth thinking about.

So where does this leave the Sol-Ark Essentials 12K-2p inverter? It still has its uses. The primary one is that the 12K comes with all the integrated breakers. So it comes with a breaker for load, a breaker for generator, a breaker for grid, and a breaker for the batteries. The larger Sol-Ark 15K only has a breaker for the load and a breaker for the batteries. That external grid disconnect with overcurrent protection, as well as the generator disconnect, aren’t included. Plus, a load-side connection to an essentials load panel doesn’t require having the utility out to site to pull the meter. The 12K is a very simple installation, with backup power capability, making it a good compromise between non-backup and whole home options. Another example of a unique 12K application is for small, distributed outdoor charging stations. It’s a very simple way to provide a little bit of outdoor 120/240V power that doesn’t require the architecture of the 15K.

At the commercial level, this Sol-Ark GEN port is rated for 200 amp, the same as the grid-pass through rating, whether it’s the 30K-208V or 60K-480V inverter. This is an incredibly generous for interconnection point but remember with great power comes great responsibility. Keep in mind these are the maximum amperage port ratings, and the continuous output is software constrained to 180A continuous.

But now we have also that same kind of port as our Gen Port. So on the residential line, the Gen Ports are much smaller. This can accommodate much larger power generation for EV chargers, for interconnected generators, building generators, three-phase generators, so much larger than what we’ve had in the past with our residential line.

But fundamentally, it’s still the same. You still have a load port for your main panel, and then you have a port for DC coupled solar. And then actually there’s two battery ports. And I think what’s this… Really, this is off topic, but the key design issue is these battery ports are 50 amps each.

And so if you want the full 100 amp battery power capability and you only have one battery cabinet, you need to run parallel conductors from the cabinet, or at some point have a short busbar. Or you get two battery cabinets. Or you get a battery cabinet that has two sets of parallel conductors. I mean, there’s couple ways to do that. The main panel is a 50 amp backup. If you have two inverters, that’ll get you to 100 amp backup. You can go all the way up to 10 inverters indoors, 6 inverters outdoors at the commercial level.

To close out this interconnection discussion, let’s finish with AC coupling, using some commercial project examples. DC-coupling is my preferred system architecture more many reasons, one of which being it keeps the GEN port available for other uses besides solar AC coupling. But commercial solar has a wide variety of low cost rooftop solar options, especially at 480V, such as the SMA Sunny Tricore. It’s valid to keep these UL3741 inverters on the commercial rootop (eliminating module-level rapid shutdown equipment), to utilized that GEN port on hybrid inverters to land the AC-coupled solar, and then to locate the hybrid inverter near either the battery location or point of interconnection.

There are a few project considerations when interconnecting AC-coupled solar. The first is to determine if solar is part of the backup project. If so, the solar AC output size should be less than or equal to the nameplate of the hybrid inverter, even if the GEN port can accommodate higher amperage. If it gets much larger than that, then the frequency shifting to turn off the solar array in backup mode is unreliable.

Similarly, zero export mode for AC couple isn’t possible. Again, zero export mode is caused by frequency shifting, and its not possible for such a small system to change the grid frequency if the grid is online. The grid frequency is going to win out unless there’s a power failure. So if you need zero export, or you need the solar array for backup power, keep any AC coupled solar to be smaller than the inverter nameplate for standard installations.

AC coupled solar can land in two locations on hybrid inverters, it isn’t necessary to AC couple solar onto the GEN port. It can be connected onto the LOAD port along with the rest of the service breakers, assuming it is otherwise code compliant. Frequency shifting will still work to control the solar array while in backup mode. What is gained by interconnecting solar onto the GEN port is dedicated solar monitoring within the hybrid inverter app, which is very convenient. Whereas if you connect solar on in with all of your other electrical loads, the data that’s reported through that exchange is mixed together.

Combining AC coupled solar and generators simultaneously is difficult. Not all hybrid inverters can do this well. Nor can it be done with all generators. In particular, large, clean “top shelf” generators which provide an electric signal similar to the grid are tough to implement, because if frequency shifting cannot work, the the solar array can backfeed the generator. It is possible to run AC-coupled solar and generators connected to the GRID port on Sol-Ark systems, but again, its better to avoid AC-coupled solar and generators all together if at all possible.

Solar is a crystal lattice – you have to remember your high school chemistry classes for this one! Whether its polycrystalline or monocrystalline, the electrons within a solar panel are stuck in this kind of glass-like molecular structure. The way solar works is photons from the sun hit that crystal and knock the electrons out of where they’re supposed to be – it’s very similar to how a cue ball disrupts billiards on a table. In our pool table analogy, the solar panels have these electron holes remaining which want to attract electrons, but a trick is employed to keep the liberated electrons. Almost like picking up the edge of a pool table, chemicals are used to convince all the electrons to collect on one side of the solar cell. The positive charge form where the electrons were combined with the negative charge to where the electrons collect is a primarily driver for the photovoltaic effect aka solar power. So long as photons from the sun are hitting the solar panel, those electrons will be bouncing around the inside of a solar cell, getting collected along the silver linings etched into the cell, and create electrical DC current.

In general, one watt of solar produces 1.4 kilowatt-hours a year, or one kilowatt of solar produces 1,400 kilowatt-hours a year. That’s a little bit more production in very sunny climates like the Southwest United States, but anywhere from say, Houston to Minneapolis, the various combinations of sunlight and temperature average out to around that 1kW to 1.4 MW ratio, at least for roof mount arrays. Keeping this ratio in mind is useful for solar mental math – an electric bill that shows 17,000 kilowatt-hours a year of usage can be quickly estimated to be offset by as a 12kW solar array for example.

This ratio can be adjusted to your local area by using the website PVWatts, funded by the government National Renewable Energy Labs, which provides this data by considering local data from nearby weather stations. Put in your address, and you get a month-by-month printout of how much energy the solar array produces. PVWatts provides a wealth of useful data beyond just energy production estimates. You can use it to explore inverter clipping, check sunny days versus cloudy days, ambient temperature, rooftop temperature, wind speed – all sorts of things that are commonly overlooked but super useful for system design. PVWatts is an industry standard, and commercial design software (with more sophisticated features) commonly uses the same dataset that PVWatts pulls its energy estimates from.

Fundamentally, solar design is about how many rectangles you can fit on a larger polygon – it’s not very difficult to design a solar array layout. Fit the right number of solar panels on the roof and plug that figure into PVWatts to get an accurate solar energy estimate. There are some design rules and rules of thumb, like keeping the solar array accessible, shade free, and good-looking. Inverter products can mitigate shade to some degree, so if the solar array is mostly sunny, aesthetic, and easy to access, you’re on your way to being a entry-level solar designer. PVWatts does a small amount of international site locations as well.

Today, solar design is undergoing an industry change. Previously there’s been this standard operating procedure of of covering the entire rooftop with solar, driven by net-metering policy. Net-metering in the USA has traditionally required the utility to buyback solar electricity from their customers at a price that is advantageous price to the customer, such that the largest projects are the most cost-effective. But net-metering is being rolled back in most of the United States, making it time to consider the value of smaller arrays.

If the idea of doing smaller systems makes solar installers nervous (as it would mean less work and money) well, the good news is that batteries are even more expensive than solar! Solar projects are supported by industry professionals because they have healthy project budgets, and even if the solar scope of the project is shrinking, the addition of batteries to a solar project keeps the project budgets within industry norms.

Plus, customers want batteries. Without batteries, solar arrays cannot provide reliable backup power during outages. Also batteries make economic contributions to the project, and can create more cost-savings than the solar array itself. As long as net-metering policies are replaced by peak and off-peak rates where the price of electricity varies based on time of day, a small solar array and battery system can significantly reduce an electric bill and provide some backup power during an outage. The most economic system might not be large enough to run an entire home for any significant period of time, but some backup power is still a lot better than none at all.

In short, residential and commercial solar installers with access to time-of-day or time-of-use metering should think about the projects being battery first, with the solar scope being as an add-on accessory option. That option might only be a small solar array on the easiest accessible roof surface, such as the garage directly above the battery. That solar array might be much smaller than what the installer might have previously considered a viable project. But today the goal is not just solar; it’s solar plus batteries.

Keep in mind, it’s not about how much energy you produce; it’s also about how much that energy is worth.

California is still a great solar market, even with net metering rollbacks. It’s very sunny in California and electricity is still expensive. Many Californians experience regular power outages. The solar market hasn’t gone away in California just because net-metering policy changed, but the project pathway to the market has changed. California is no longer a solar-only market; it’s a batteries-first market.

In contract to sunny California, the northeast coast enjoys its seasonal weather. But the northeast also has high electricity pricing, and it is also enjoys a robust consumer-owned solar market. The price of electricity varies more across the country than the amount of sunlight, so it is fair to assume that the price of electricity is the primary reason consumers move forward with a solar purchase. Backup power plays a secondary role, and multi-day whole home backup isn’t always in the best interest of the customer. Payback is what sold solar power in net-metering markets, and it attracted a larger customer base than customers prioritizing backup power.

The problem with today’s solar market is that the economics have now become extremely complex. Solar payback calculations used to be possible on a paper napkin – you cost savings equaled how much energy you produced multiplied by that net-metered rate. If you generated 80% of your home energy, your electric bill would reduce around 80%, regardless of whether it was consumed on site or sold back to the grid. There’s a whole discussion to be had about the role of simplicity in residential rate structures, and the number one argument in favor of net-metering as being the ultimate renewable policy is that homeowners have traditionally enjoyed the right to simple, metered electricity plans. The right to simple, metered electricity is eroding on many fronts, but we don’t have time to talk about that in today’s class. It’s much more complicated to calculate payback in today’s market. Since this is an intro class, let’s set aside those economics for now and talk about solar design. We’ll return to economic discussion later in this class.

Fire code rules found in building code govern array layouts, even if not enforced in all areas. A rule of thumb for residential array layouts is staying at least three feet away from the top and sides of the building, but leaving even more space is useful. The idea is that firefighters need access to the roof to swing an axe through it, if the area below is filled up with smoke. This access space has other benefits. Installers will need to get back up on the roof after installation. Wind speed is increased at the roof edges as well. All of this is considered when planning rooftop attachment points discussed later.

Commercial buildings have their own sets of setbacks – it’s more than residential, with greater spaces from rooftop edges, as well as walkway requirements and even how to orient the solar panel to minimize failure during a wildfire. Importantly, solar arrays must be broken up every 150 feet, although I think stopping sooner is a little better. Don’t forget to put expansion points in the conduit runs breakouts are there to help inhibit the impact of thermal expansion on your conduit, which is a common cause of rooftop solar fires. Fire protection systems installed within the solar array do not easily detect this kind of failure. Again, we will have more on racking components later.

Other important site evaluation questions can be answered by detailed pictures of the electric service panel, as well as “far away” photographs where the electricity enters the building, and perhaps a picture of the utility meter. That data can be useful when considering interconnection technique, which we will need another class to discuss. My residential preference is for hybrid inverters with 200A passthrough, using 48V battery banks, the largest of which is the Sol-Ark 15k. This is because the inverter is simply installed between the utility meter and the 200A service panel, providing everything needed for a solar, battery, and whole home connection, along with an additional port for a generator, EV charger, AC solar, or extra sub-panel with some added value in smart control for whichever device is attached. That keeps the site simple to design, easy to install, and flexible to accommodate any peculiarities.

Regardless of the inverter type, it is important to know the number of solar panels which can fit on a circuit. The Sol-Ark 15k is a string inverter with multiple MPPT, meaning that circuits are traditionally wired in series to form high voltage DC circuits with some tolerance for shaded rooftops. String inverters are great for sunny roofs and can be very shade tolerant in the hands of an expert, but that has not always been the case. Back when string inverters only has a single shade tracker, Solaredge created a DC Optimizer that put shade tracking at the module level. This also boosted the system voltage, so that more modules could be landed on a single circuit before landing on the string inverter below. Enphase took a different approach, eliminating the inverter on the side of the building, and making a micro-inverter, which was also installed behind every single solar panel on the roof, but producing 120V AC the same as what is used in USA homes. Out of these three inverter types, even when you include the various battery components, Enphase solution is the most expensive, and string inverters with rapid-shutdown only systems at the module or string level remain the lowest cost technical solutions on the market, at least, until the value of the manufacturer is added into the calculation.

Rapid shutdown is the ability for firemen to push a button (which can be a lockable button) outside the building and shutdown the solar array voltage on the roof. It took over a decade for the listing to be published after it was first implemented in National Electric Code, and in that time, SolarEdge and Enphase nearly a full monopoly on the market. By putting their computers behind every solar panel on the roof, they could abide with rapid shutdown requirements without having a listing. While ten years late to the party, the new UL3741 racking-RSD co-listing allows for string-level disconnection, which not only is a lower cost solution to rapid shutdown, while simultaneously eliminating the need for all those computers on the roof. So I fully expect the market to shift toward hybrid inverters with string-level rapid shutdown in the years to come, such as solutions being explored by Solodeck and Midnite Solar.

String-sizing, or the ability to calculate the right solar circuit for the job, is performed by manufacturer provided string sizing tools. It’s important to learn the calculations for these various tools, whether it be Enphase, SolarEdge, or Sol-Ark. But if beginning solar design, its also helpful to learn these tools, as well as those found in commercial software like Aurora Solar. There are even tools that generate complete permit packages, like Lyra (formerly SolarDesignTool). Lastly, handmade design packages are also inexpensive. Arka360, Aurora Solar, and Enact are examples of industry design software which fits in a wide variety of design and permit services into their commercial offers.

By and large, if you are unfamiliar with solar module selection, just visit some distributor websites and you will many options. A popular option is an “all-black” module, and there is even a range in quality for that feature. Its important to buy from quality solar manufacturers, but it is not important to buy the most efficient solar panel on the market, unless you have net-metering or a small rooftop.

Solar racking design warrants its own class, but the short of it is there are solar racking products for every rooftop, even if it isn’t a good idea. Like inverter manufacturers, many racking manufacturers have design tools on their website. I admire IronRidge for their leadership in this, because their design tools provide significant data, such as load diagrams and easy spacing requirements for those roof attachment points. So long as you abide by those spacing requirements, staggeting the attachment points across the roof rafters as uniformly as possible, and giving the the corners extra reinforcement (as well as bottom rows of rail in areas of heavy snow), these systems are fairly straightforward to install. There’s even rail-less racking systems for rooftops where you don’t have a clear picture of what’s underneath.

It would be nice to explore these topics in a bit more detail, so perhaps a part II of this introduction to solar is in order. In the meantime, thanks for being a good audience member!

When it comes to choosing batteries for your solar and storage system, it’s not always as simple as it seems. You might initially think, “Wouldn’t it be great to have the battery and inverter from the same manufacturer?” It sounds convenient, right? Certainly installers prefer having two brands together, knowing they can rely on a single company like for support with both battery and inverter issues. However, while having an integrated battery and inverter may appear simpler at first glance, there’s a crucial factor to consider: the end-of-life implications for your batteries.

Simplicity in solar battery design is often a good thing – it’s a driving force behind solar design. But there are long-term challenges that arise when the battery and inverter are too closely tied together. Consider this analogy: when the batteries in your TV remote die, you simply remove the old ones, dispose of them properly, and pop in some new batteries. You don’t toss out the entire remote and buy a new one just because the batteries are depleted. The same principle applies to your solar and storage system.

The reality is that batteries have a shorter lifespan than inverters. So, if you’re locked into a specific battery brand or model that’s directly integrated with your inverter, you might find yourself in a tough spot 10-12 years down the line. It’s not about whether the manufacturer will still be around – it’s about whether they’ll continue to support that specific product. Technology evolves rapidly, and if your inverter is designed to work only with a particular battery that’s no longer supported, you could be forced to replace the entire system, even if the inverter itself is still functioning perfectly.

That’s where battery choice comes in. By opting for a battery-agnostic or battery-choice platform, you give yourself the flexibility to adapt as technology advances. When the time comes to replace your batteries, you can select the best option available based on factors like cost, features, and compatibility. This approach ensures that your inverter remains viable, and you can continue to benefit from your solar and storage investment without unnecessary expenses.

Now, let’s talk about cost. There’s no denying that batteries are the most expensive component of your system – they’re the part that drains your budget the fastest. Everyone dreams of living off-grid, relying solely on solar and batteries, but the reality is that the cost of batteries alone often makes that impractical without additional considerations like a backup generator or careful load management. So, if you’re aiming for the lowest possible project cost, where do you turn?

The answer lies in the history of lithium-ion batteries. Initially, these batteries were used to power server rooms, and the most cost-effective form factor was the server rack battery – a simple box with the cells built into it. While server rack batteries like those offered by Pytes might require a bit more labor during installation due to additional wiring and communication setup, they ultimately provide the most bang for your buck.

That being said, there are some high-quality server rack batteries that defy the norm. Blue Planet offers an expensive server rack option but combined with an industry leading warranty and incredibly high cycle count. Storz also makes a server rack battery with a solid value proposition and a strong focus on warranty terms and conditions. The key takeaway is that there are a diverse range of features to suit different needs and budgets, even within a generic server rack form factor.

A more advanced battery feature is stackable batteries. They’re a bit pricier than standard server rack batteries, but they offer easier installation and built-in structural elements that streamline the wiring process. HomeGrid, for instance, features batteries that simply stack on top of each other and plug in like power tools, with a BMS unit on top and conductor runs coming out of the unit. Pylontech’s Pelio battery is another stackable battery option. While it does not have integrated stackable battery connectors, it has a very easy wiring cabinet running down the side of the battery stack, and its upright battery design allows it to hug the wall closely, making it incredibly space-efficient. The lifting handles built into the case make it easy to lift batteries. Combined with the easy access wiring compartment, its one of the most user-friendly batteries I’ve encountered. Small details like the two-wire screw connector for communication, rather than an Ethernet connector, make the installation process smoother and more reliable.

EndurEnergy is another battery pack manufacturer which stands out for its versatility in cabinet design, voltage options, and EMP hardening features. They offer a battery which can be used in both low-voltage (48V) and high-voltage applications. This flexibility is particularly valuable for installers looking to simplify their inventory and accommodate different project requirements. Their wide range of cabinet options span various scenarios, including cabinets which house both the inverter and batteries. While most residential installations don’t require a cabinet for both the inverter and batteries, there are situations where it’s beneficial. For example, small EV charging stations in parking lots or publicly accessible spaces with limited power access can benefit from a self-contained setup that keeps the equipment protected from tampering. Endure’s thin profile cabinets are another notable offering, designed to fit seamlessly in tight spaces like the narrow section between the garage wall and the door. These cabinets provide a clean, bundled solution without exposed cables, making them an attractive option for homeowners who prioritize aesthetics and space-saving designs.

In addition to their diverse cabinet options, Endure is offers EMP (electromagnetic pulse) hardening battery options, popular when pairing with Sol-Ark EMP-hardened inverter models. Tested to military standards for EMI resilience, an EMP-hardened battery can only be achieved with use of high-quality internal components within the battery management system. For homeowners who want to be prepared for any scenario, an EMP-hardened battery and inverter combo from Endure and Sol-Ark is a compelling choice.

One advantage of high-voltage batteries is the use of series wiring instead of parallel wiring. In general, voltage is cheaper than amperage, meaning that high-voltage power lines are more cost-effective per watt than low-voltage lines. The smaller cable gauges associated with high voltage are also cheaper and easier to work with compared to the larger gauges needed for high-current applications. However, it’s important to note that high-voltage batteries are not necessarily cheaper than low-voltage batteries at this point in time. They are less widely available, and there’s a higher potential for issues to arise in a high-voltage system due to the series wiring configuration. In a parallel system, one faulty battery may not bring down the entire bank, whereas a single point of failure in a series system can have a more significant impact. To address the specific needs of commercial projects and the challenges associated with high-voltage systems, Sol-Ark has developed a line of high-voltage batteries to pair with its commercial inverters. So perhaps battery choice is a more valuable feature for residential systems, which need maintainable upgrade paths when parts whereout, as opposed to commercial systems, where the critical nature of commercial backup power and the complexity of high-voltage configurations might warranty a single brand solution.

Now, let’s dive into the inner workings of a battery management system (BMS). It’s surprising that the NABCEP Energy Storage Installation Professional exam and associated documents like NFPA 855 and NEC 706 don’t delve into the intricacies of BMS components. Understanding the role and function of these components is crucial, much like how a high-end stereo’s sound quality depends on the quality of its internal parts.

At a fundamental level, there are two types of battery management systems: those that rely solely on PCB (printed circuit board) components like MOSFETs to manage energy flow, and those that incorporate contactors for higher amperage handling. MOSFETs, commonly found in solar optimizers, microinverters, and rapid shutdown devices, are well-suited for managing low-current, high-voltage applications like individual solar panels. However, they have limitations when it comes to handling the high currents often encountered in low-voltage battery banks.

MOSFET-based BMS are less expensive but are only rated to handle the current of a single battery. In a scenario where one battery in a bank is significantly more discharged than the others, attempting to charge that battery can result in excessive current flow from the fuller batteries to the depleted one. This can lead to BMS failure, as the MOSFETs are not designed to handle such high currents. Consequently, the system may require maintenance, troubleshooting, and potential component replacement.

On the other hand, contactor-based BMS, like those found in electric vehicles and many high-quality stationary battery systems, are capable of managing much higher amperage than MOSFET-based systems. Contactors are essentially heavy-duty relays that can handle the high currents associated with low-voltage battery banks. However, this increased current handling capacity comes at a higher cost and with a slightly higher parasitic load on the battery.

The parasitic load of a contactor-based BMS is worth considering, especially in systems where each battery module has its own BMS. While a single BMS managing an entire battery bank may not have a significant impact on overall efficiency, the cumulative parasitic load of multiple contactor-based BMS can be more noticeable. In a 5 kWh battery, for example, an inefficient BMS could potentially drain 10-15% of the battery’s capacity over the course of a day when combined with the inverter’s power consumption and other factors.

Despite the higher cost and parasitic load, contactor-based BMS are often the preferred choice for their robustness and reliability. The automotive industry, in particular, has embraced contactor-based systems due to their ability to handle high currents and maintain low failure rates. Additionally, the slight inefficiency of contactor-based BMS can actually be beneficial in cold weather, as the waste heat helps keep the batteries warm and at an optimal operating temperature.

Ultimately, the choice between MOSFET-based and contactor-based BMS comes down to the specific requirements and budget of the project. For the absolute lowest cost, a MOSFET-based system may be the way to go, but it requires careful commissioning to ensure all batteries are at similar states of charge to avoid overloading the MOSFETs. Contactor-based systems, while more expensive, offer greater peace of mind and long-term reliability.

Another crucial aspect of battery system design is understanding the short circuit current rating, also known as the available fault current. This rating is often overlooked in standard battery certification exams but is essential knowledge for any battery professional. A battery’s short circuit current can be significantly higher than its continuous output rating, and it’s typically regulated by an internal overcurrent protection device, such as a fast-acting fuse.

When a battery is shorted directly to ground, it can deliver an enormous amount of current in a very short time, potentially causing severe damage to the system. High-end BMS often incorporate fast-acting fuses rated for much higher amperage than the battery’s continuous output to protect against these scenarios. For example, a 100 Ah battery might have a 300 A fuse, which may seem counterintuitive. However, these fuses are designed to blow within milliseconds in the event of a short circuit, limiting the available fault current and preventing catastrophic failure.

Proper fuse sizing is particularly important when paralleling multiple batteries together, as the available fault current becomes cumulative. The bus bars connecting the batteries must be rated to handle the combined fault current of all the batteries in the system. In some cases, additional external fusing may be necessary to limit the fault current and protect the system components. It’s crucial to consult the battery manufacturer’s data sheets and guidelines when designing these systems to ensure proper fuse selection and coordination.

Pre-charge relays are another important component of a battery management system that installers should be familiar with. When a battery is first connected to an inverter, especially a large inverter with empty capacitors, there can be a significant inrush of current as the capacitors charge up. This inrush current can potentially damage the battery or trigger overcurrent protection, preventing the system from starting up properly.

To mitigate this issue, many high-end battery systems incorporate a pre-charge relay that allows a small trickle of current to flow into the inverter’s capacitors when the battery is first turned on. This gradual charging process helps to minimize the inrush current and allows the system to start up smoothly. Once the capacitors are sufficiently charged, the main contactor closes, allowing full current to flow between the battery and the inverter.

The pre-charge relay can also be used to recover a deeply discharged battery. In some cases, a battery that has been severely depleted may have its BMS lock out charging and discharging to protect the cells from further damage. To resolve this, a knowledgeable installer can use the pre-charge relay to slowly charge the battery back up to a safe voltage level, at which point the BMS will unlock and allow normal operation to resume. This process may involve repeatedly power cycling the battery and allowing small amounts of charge to trickle in until the voltage threshold is reached.

Having a dedicated 48V lithium battery charger on hand can be a valuable tool for installers, particularly those working with 48V residential systems. These chargers, available from companies like Aims for a few hundred dollars, provide a low-amperage trickle charge that can be used to gradually bring a deeply discharged battery back to life. For those on a budget, an electric bike charger with alligator clip adapters can serve a similar purpose, as long as it’s rated for the appropriate voltage range.

Now, let’s take a closer look at an the most recent server rack battery offered by Pytes. The battery incorporates several familiar features, such as a toggle switch for battery-to-inverter communication, communication ports, and positive and negative terminals. However, one standout feature is the inclusion of a dry contact, which is typically used to override the BMS in cases where the battery has been deeply discharged and locked out. This dry contact allows a skilled technician to bypass the BMS’s safety mechanisms and charge the battery back up to a usable level, potentially saving the customer from having to replace the entire battery pack.

While it may be tempting to use the dry contact for remote on/off functionality, similar to a rapid shutdown system for solar arrays, I would caution against this approach. Rapid shutdown requirements exist for solar panels because they are often located on rooftops, far from the inverter, and may have long conductor runs through attics and walls. Disconnecting power close to the source is important in these cases. However, batteries do not face the same challenges, as they are typically installed in close proximity to the inverter and are already equipped with built-in disconnect switches.

Moreover, using the dry contact for remote shutdown could potentially introduce unnecessary risks and complications. Repeatedly power cycling the batteries to force a shutdown could lead to unintended consequences, such as blowing fuses or damaging components if the batteries are at different states of charge. It’s important to remember that the NEC already has voltage thresholds in place for battery systems, requiring any serviceable parts to be below 100 V, which a 48 V system inherently satisfies. While some competitors may market a “rapid shutdown” feature for batteries, it’s crucial to recognize that this is not currently a code requirement, nor should it be. The existing battery disconnect switches and voltage limits provide sufficient safety measures. Overengineering a solution to a problem that doesn’t exist can lead to increased costs and potential failure points.

Now, let’s shift our focus to the different types of battery cells used in lithium-ion batteries. The three most common form factors are pouch cells, cylindrical cells, and prismatic cells. Pouch cells, like those found in laptops and cellphones, are lightweight and compact but lack the structural rigidity of other cell types. Cylindrical cells, such as the familiar AA or 18650 format, are known for their high energy density and good thermal management but are less commonly used in stationary storage applications.

Prismatic cells, on the other hand, have become increasingly popular in both electric vehicles and stationary storage systems. Despite the somewhat intimidating name, a prismatic cell is essentially just a rectangular prism-shaped battery, similar to a traditional lead-acid car battery. The key advantage of prismatic cells is that they offer a good balance between the high energy density of pouch cells and the mechanical stability of cylindrical cells. This makes them well-suited for applications where vibration, impact, or compression may be a concern.

When comparing the different cell types, it’s important to consider the specific requirements of the application. Pouch cells are often the most cost-effective option, but their lack of built-in structural support means that the battery pack or module design must account for this. Cylindrical cells are highly standardized and have well-established manufacturing processes, which can make them easier to source and integrate into a system. However, their shape may not always be optimal for maximizing energy density in a given space.

Prismatic cells, with their rectangular shape and sturdy construction, offer a good compromise between cost, energy density, and mechanical resilience. They can be efficiently packed into a battery module or pack, and their flat surfaces make them easier to cool than cylindrical cells. However, prismatic cells may be more expensive than pouch cells due to the additional manufacturing steps required to produce the rigid casing.

When it comes to selecting the best battery for a given application, the operating environment plays a crucial role. In a temperature-controlled server room, for example, a pouch cell battery may be the most cost-effective option and therefore the best choice. Historically, cylindrical batteries have been cheaper than prismatic cells, but the industry has been trending towards prismatic designs due to their superior durability.

The load profile of the application also has a significant impact on battery selection. A residential home, for instance, has a much more volatile load profile compared to a stable server room. Residential systems often lack the sophisticated heating and cooling systems found in electric vehicle battery packs, which makes prismatic cells the preferred choice for stationary storage in residential settings.

One of the key advantages of prismatic cells is the ability to monitor the voltage of each individual cell. In a 16-cell prismatic battery module, for example, there are individual cables connected to each cell, allowing the battery management system (BMS) to read the voltage of each cell independently. This level of granularity enables the BMS to detect and flag issues like a single cell with abnormally low voltage, which could indicate a dead or failing cell.

Prismatic cells are typically larger in form factor than cylindrical cells. A 5 kWh battery pack might contain 100 cylindrical cells, whereas the same capacity could be achieved with just 16 or even 15 prismatic cells. This larger form factor has several implications, including the minimum room size required for fire safety testing.

When a large prismatic cell undergoes thermal runaway and catches fire, it releases a greater volume of gas compared to a smaller cylindrical cell. This can be misleading, however, as the thermal runaway in a lithium iron phosphate (LFP) battery is typically confined to a single cell or group of cells, unlike the lithium chemistry made popular by Tesla, lithium nickel manganese cobalt (NMC), where the thermal runaway can easily propagate from cell to cell throughout the entire pack. This distinction in thermal runaway behavior is a key safety advantage of LFP batteries over NMC. It’s one of the main reasons why LFP has become the dominant chemistry in stationary storage applications, particularly in residential settings where safety is of utmost importance.

The ability to monitor individual cell voltages is another significant benefit of prismatic cells. While this information may not be directly accessible to the installer or the end-user, it provides valuable data for the manufacturer when assessing warranty claims. With cylindrical cells, it’s not feasible to have a voltage sensor on every single cell, so the BMS can only monitor the voltage of groups of cells rather than each cell individually.

This difference in monitoring resolution can have important implications for detecting and diagnosing cell degradation issues. With prismatic cells, the BMS can clearly identify when a specific cell has failed or is degrading abnormally. With cylindrical cells, the BMS may only be able to detect a general reduction in overall pack capacity, without being able to pinpoint the specific problematic cell or cells.

In the context of warranty claims, this distinction is crucial. If the BMS can’t identify a specific cell failure, the manufacturer may not be aware that there is a valid warranty claim to be made. The reduced pack capacity might be mistaken for normal degradation rather than a premature cell failure. With prismatic cells and individual cell monitoring, these issues are much easier to detect and diagnose.

The difference between normal cell degradation and a genuinely faulty cell can be very difficult to determine, even with individual cell monitoring. A skilled technician with a voltmeter may be able to identify a problematic cell more easily in a prismatic pack compared to a cylindrical pack, but it still requires careful analysis and interpretation of the data.

As shown in this chart, a key differences between LFP and NMC chemistries is nominal cell voltage, with LFP having a lower voltage per cell compared to the other chemistries shown. This difference in cell voltage has important implications for the performance and safety characteristics of the battery. If you’re designing a high-performance electric vehicle, such as a sports car that will only be driven occasionally but needs to accelerate from 0 to 60 mph in under 4 seconds, you might choose NMC cells to take advantage of their higher cell voltage and corresponding power density. With NMC, its possible to push the cells harder. They also burn more brightly in other senses, wearing out faster as well as coming with lower activation temperature for thermal runaway.

If you were to put an NMC battery pack in your oven and turn the dial to the maximum, the cells would quickly go into thermal runaway and catch fire. An LFP battery, on the other hand, would be much more resistant to thermal runaway under the same conditions, requiring the oven setting to get all the way to broil.

To illustrate this point, let’s look at a comparison of two pouch cells undergoing the nail penetration test, which is a standard safety test required for UL certification. In this test, a conductive nail or spike is driven into the cell to simulate an internal short circuit. When the nail pierces the NMC cell, we can see a significant amount of gas venting from the cell, followed by ignition and sustained burning. This is classic thermal runaway behavior.

In contrast, when the same test is performed on an LFP cell, the results are dramatically different. While the nail penetration still causes the cell to short internally and vent some gas and smoke, the cell does not ignite or go into sustained thermal runaway. Even when the nail is left in place to maximize the heat generation from the short circuit, the LFP cell may get hot and vent, but it doesn’t reach the critical point of thermal runaway and self-ignition.

This behavior is further illustrated in the UL 9540A fire test, where cells are subjected to external heating to intentionally induce thermal runaway. For an NMC cell, the heat from one cell in thermal runaway is sufficient to cause adjacent cells to also go into thermal runaway, leading to a cascading failure that can consume the entire battery pack and spread to neighboring packs. With LFP cells, the heat generated by one cell in thermal runaway is not enough to ignite the next cell, so the failure is inherently limited in scope.

This inherent resistance to thermal runaway propagation is the key safety advantage of LFP batteries, and it’s the main reason why they have become the preferred choice for stationary storage applications, especially in residential settings. Even in the event of a fire or other catastrophic failure, an LFP battery is much less likely to contribute to the spread of the fire or cause secondary ignitions.

Another important aspect of battery system integration is the choice of communication protocol between the battery and the inverter. The two most common protocols are CANbus and Modbus. Many battery manufacturers will use CANbus to communicate with the inverter, as it allows the use of Modbus port for other devices such as energy meters, home automation systems, or inverter controllers which are traditionally using Modbus protocol.

When wiring battery communication to an inverter, it’s important to carefully review the pinout of the communication ports on both the battery and the inverter to ensure compatibility. In some cases, a custom communication cable may be required to properly connect the two devices. While some battery manufacturers include the necessary communication cable with their products, installers should always be prepared to fabricate a cable on-site if needed.

Having the right tools and supplies on hand is essential for any battery installer. A reliable set of cable crimpers, including a hydraulic crimper for larger gauge wires, can make field terminations much easier and more secure. Investing in a quality punch kit with a variety of die sizes can also be helpful, particularly when dealing with conduit or enclosure entries.

In addition to basic electrical tools, installers should also have a few specialized items in their kit. A CAN bus analyzer or diagnostic tool can be incredibly valuable when troubleshooting communication issues between the BMS and the inverter. Similarly, a dedicated 48 V battery charger can be a lifesaver when dealing with a deeply discharged battery that needs to be carefully brought back up to a safe voltage before being connected to the system.

When it comes to battery cabling, it’s important to use properly rated wire that is suitable for the expected current and voltage levels. In most cases, fine-stranded cable is preferred over solid wire due to its increased flexibility and resistance to fatigue. However, it’s crucial to be aware of the ampacity derating that applies when using fine-stranded wire in conduit. NEC ampacity tables for flexible cabling only apply to wires located outside of conduit, and additionally, NEC general wiring rules require ampacity derating of conductors in raceway when there are more than three current-carrying conductors in the conduit. This is often referred to as the “more than three rule” and is intended to account for the increased heat buildup that occurs when multiple conductors are bundled together. In practice, this means that if you have a four-conductor battery cable in conduit (two positive and two negative), you must size the conductors as if they were only rated for 80% of their nominal ampacity according to the general rather flexible wiring ampacity tables. Failing to account for this derating can lead to overheating and potential fire hazards, so it’s essential that installers understand and follow this rule.

Another important consideration when working with battery systems is the proper use of overcurrent protection devices (OCPDs). Every battery system should have a means of quickly disconnecting the battery from the rest of the system in the event of a fault or short circuit. This is typically accomplished through the use of a circuit breaker or fuse that is rated for the maximum expected fault current of the battery.

When sizing the OCPD for a battery system, it’s important to consider both the continuous current rating and the interrupting rating. The continuous current rating must be greater than or equal to the maximum continuous discharge current of the battery, while the interrupting rating must be sufficient to safely clear the maximum fault current that the battery can deliver.

In some cases, the BMS may provide an additional layer of overcurrent protection through the use of internal fuses or contactors. However, these devices should be viewed as supplementary to, rather than a replacement for, a properly sized external OCPD. The external breaker or fuse serves as the primary means of protecting the battery and the connected equipment from damage due to overcurrent conditions.

Battery warranties are another critical factor that installers and system owners must carefully consider. Most lithium-ion battery manufacturers offer a 10-year warranty that guarantees a certain level of remaining capacity at the end of the warranty period, typically around 60-80% of the original rated capacity. However, some manufacturers are now offering extended warranties of up to 25 years, often through third-party insurance providers. While a 25-year warranty may sound attractive, it’s important to understand the terms and conditions of these extended coverage plans. In many cases, the guaranteed end-of-life capacity is significantly lower than what is offered under the standard 10-year warranty, often around 40-60% of the original capacity. This means that while the battery may still be functioning after 25 years, its usable energy storage capacity will be greatly reduced.

Moreover, these extended warranties often come with strict requirements for battery usage and maintenance, such as adhering to a certain depth of discharge (DoD) limit or performing regular capacity tests. Failing to comply with these requirements can void the warranty, leaving the system owner responsible for the cost of any repairs or replacements. As with any long-term investment, it’s essential to carefully weigh the potential benefits and risks of an extended battery warranty. For some system owners, the peace of mind and potential cost savings may be worth the additional upfront expense. For others, a standard 10-year warranty may be sufficient, particularly if the battery is expected to be replaced or upgraded before the end of its useful life.

Regardless of the warranty term, it’s crucial that installers and system owners maintain detailed records of the battery’s performance and any maintenance or repair activities. This documentation can be invaluable when filing a warranty claim or troubleshooting issues with the system. Most battery manufacturers provide some form of monitoring software or portal that can be used to track key metrics like state of charge, cycle count, and temperature.

Temperature is the last subject of this article. The simple fact is that all industry data sheets are misleading with regard to battery temperature. The problem is the operating temperature range of battery chemistry is not the same, nor even close to, its ideal temperature range. Yes, lithium batteries work outdoors in the Texas heat, up to 130 degrees Farenheit. And yes, in the wintertime, lithium batteries work in cold temperatures – charging in near freezing conditions and discharging even below freezing., there’s a range where you can continue to discharge the battery.

But what I want to point out is that, like solar panels, batteries are rated at a standard test condition of 77°F (25°C), which is a lot different than 130°F (55°C) or 32°F (0°C). The battery cycle rating and throughput warranty are also calculated assuming an near standard test condition for normal operation. But most residential batteries are not air-conditioned – they’re put outside, subject to the heat and cold. So there is a concern that the industry forgets this fact, amongst the cycle warranties, and the reinsurance with ever longer warranty terms, and drinks its own Kool-Aid so-to-speak. Without a doubt, batteries will have lower performance out of the battery in extreme heat and extreme cold.

There are batteries – Pytes with their newer battery model and HomeGrid options as well – that have heaters built into their batteries. If the site regularly getting down to near freezing or below, heating the battery somehow is important. Whether its simply putting the batteries indoors or wrapping in an electric blanket or incorporating a cabinet heater, keeping the batteries above freezing will extend its life and performance. Will the heaters take up energy? Yes! But there’s not real difference between that and not having the battery to use if it cuts off below freezing, or even worse, if it degrades because it is too cold! Battery manufacturers try to strike a balance by putting freezing as the cut-off point, but fundamentally there isn’t anything magic to a battery about near freezing or at freezing. It’s not water inside the battery that is at issue.

Again, not many batteries have heated battery options on the market, at least for right now. That’s brings us back to our original topic of the importance of battery choice is important. If you’re in Los Angeles, solar battery design is fundamentally different than Wisconsin. In LA, you might get the cheapest server rack battery possible, put it outside, and be fine. Not so in Kansas! This temperature consideration is a critical factor that often gets overlooked in battery system design. While manufacturers may advertise a wide operating temperature range, it’s important to remember that the battery’s performance and longevity are rated at a specific, ideal temperature.

Deviating too far from this optimal range can have significant consequences for the battery’s health and capacity over time. Simply relying on the battery’s stated operating range is not enough – active measures must be taken to ensure that the battery stays within an acceptable temperature window, particularly in extreme climates. In milder regions with minimal temperature fluctuations, a basic battery enclosure may be sufficient. But in areas with harsh winters or scorching summers, more advanced temperature control measures are likely to be necessary. This is where having a diverse range of battery options can be incredibly valuable. Installers can tailor their designs to the specific needs of each customer and location.

At the same time, it’s important to educate customers about the realities of battery performance and the importance of proper temperature management. Many consumers may not be aware of the impact that extreme temperatures can have on their battery system, or the steps that can be taken to mitigate these effects. By having frank and informed conversations about these issues upfront, installers can help to set realistic expectations and ensure that customers are making informed decisions about their energy storage investments.

Ultimately, the goal is to design and install battery systems that will provide reliable, long-lasting performance, even in challenging environmental conditions. By staying informed about the latest battery technologies and best practices for temperature management, installers and system designers can help to push the industry forward and deliver the kind of high-quality, sustainable energy storage solutions that customers increasingly demand. While it may be tempting to focus solely on factors like price, brand name, and installation ease, the reality is that there are many battery features which impact on the long-term performance and value of the system.

As we’ve seen throughout this discussion, battery storage systems are complex and multifaceted, with a wide range of factors that must be carefully considered in order to achieve optimal performance, safety, and longevity. From cell chemistry and form factor to BMS architecture and communication protocols, every aspect of the system can be thoughtfully designed and integrated to meet the specific needs of the application.

For installers and system integrators, this complexity can be daunting, particularly as new technologies and standards continue to emerge at a rapid pace. However, by staying informed about the latest developments in the field and working closely with trusted manufacturers and suppliers, it is possible to navigate this complexity and deliver high-quality, reliable battery storage solutions that meet the evolving needs of customers and also set them up for future success.