PVWatts is a free solar performance calculator from the U.S. Department of Energy's national laboratory in Golden, Colorado, formerly NREL and now the National Laboratory of the Rockies. It combines typical-year weather data for a location with the system's size, tilt, direction and losses to estimate how many kilowatt-hours the system will produce each month. Installers, utilities and researchers use it as a neutral baseline. Our city pages use PVWatts estimates at each city's coordinates for a standard south-facing roof array; your own roof may produce more or less depending on its direction, pitch and shade. You can run PVWatts yourself for your address and enter the result in our calculator.
Related terms and pages
- Kilowatt (kW) vs. kilowatt-hour (kWh): kW measures power (system size); kWh measures energy (what you produce and use).
- Capacity factor: Actual output over a year divided by what the system would make running at full power all year.
- Degradation rate: How much a solar panel's output declines each year as it ages.
- How we use PVWatts