Update (August 2026): NREL has since been renamed the National Laboratory of the Rockies (NLR) by an administration that has trouble even uttering the words renewable energy. The calculator survives at its new address, pvwatts.nlr.gov, and the links below have been updated.
If you’re interested in solar energy and want to see your tax dollars at work to provide something useful, look no further than the PV Watts Calculator. Brought to us by what was formerly known as the National Renewable Energy Laboratory (a name that actually made sense as part of the US Department of Energy), PV Watts allows you to plug in your planned solar string/array data and your address, and get a very reasonable estimate of your solar production for just about any point in the year, and over the course of a year. It even uses historical weather data to factor in things like cloudy days, storms, snow, etc.
You can adjust the direction your panels are facing (called the azimuth), the watts your panels are supposed to produce, their angle of tilt, and other factors. I’ve found PV Watts to be slightly conservative, which I greatly prefer to something prone to overestimation.
As I write this, the current administration is on something of a slash and burn bender when it comes to government programs. And while I certainly think we need to shrink government, the administration’s approach hasn’t made much sense to me thus far, and I fear that useful efforts like the NREL and this calculator will fall prey to the current chainsaw. We shall see, and hopefully by the time you try to use it, it’s still there, and untouched.