PVWatts and the “Golden Ratio” of Solar Performance

PVWatts is a great tool for solar performance estimation, or at the very least is free and good enough for unshaded rooftops in the United States. We’ll use PVWatts to generate the “Golden Ratio” of solar to perform solar energy estimates and budget calculations in your head.

Ultimately, we want to determine how much energy a 1W solar array will produce in a year?


1 DC kW ~= __________ kwh/yr

1 DC Watt ~= _________ kwh/yr

The next step is to input your site conditions. Change the array size to 1kW from the default value of 4kW. Change the array type to “roof mount”.  Leave everything else the same for the time being.

PVWatts has advanced parameters to help you fine tune your model. Default values in PVWatts are conservative. For now, we’ll use the default PVWatts numbers under the motto, “It’s better to under-promise and over-deliver.” But even a more accurate model, annual weather patterns can cause solar performance to vary by over 10%, per year.

So if you are conservative on your design, PVWatts is all the computing power you need for unshaded jobsites.  However, as the projects grow in size, project financing requirements may request a more accurate model which should only increase the energy performance estimate number computed by PVWatts default values.

The major downside of PVWatts is that it does not model shade. For quick shade analysis, we recommend Folsom Lab’s Helioscope which has a hassle-free trial account. But guess what? Both the commercial software and PVWatts pull from the same weather data, so for unshaded, residential jobsites, PVWatts is all the computing power you need.

The final screen is an annual output for PVWatts.  We’ll produce about 1,290 kwh per year! In addition, we get useful hourly site data which we discuss in further in class. But for now, let’s go back and model a south-west facing array at the same site location. Change the 180 azimuth orientation to 225 degrees.

Notice the performance difference between a south and south-west facing array really isn’t that much. There are some minor economic implications here, but let’s continue to explore our  performance by modeling a due east facing array.


We lose about 15% of our production when reorienting from a south to an east or west array orientation. In fact, if we spun the solar array around 180 degrees and to face it north, we lose about 30% of our production. There is more to this story when you factor in economics. Always keep in mind:

It’s not just about how much electricity you produce, it’s also how much that electricity is worth.

It’s worthwhile to evaluate all project surfaces for solar. In most cases, fitting a larger array on the roof makes better economic sense than a smaller array. Installing an entire pallet of solar modules is more cost-effective than installing a smaller array.

But for solar projects oriented between southeast, south, and southwest, the energy performance estimate doesn’t budget. When covering the entire roof (moving onto east, west, and north roof surfaces), your total roof estimate “per watt” won’t shrink by more than 15% (at 4:12 or 5:12 roof tilts).

This brings us back to the Golden Ratio of Solar for Philadelphia!


1 DC kW ~= 1290 kwh/yr

1 DC Watt ~= 1.3 kwh/yr