Quick definition
PVWatts is one of the most widely used solar production estimation tools in the US, developed and maintained by NREL, the federal government's renewable energy research laboratory. Anyone can run the PVWatts calculator directly: enter a location, system size, panel orientation, and tilt angle, and it returns an estimated annual and monthly energy production figure, built from decades of historical weather station and satellite-derived irradiance data for that specific location.
Because it's free, publicly available, and grounded in solid meteorological data, PVWatts has become a common baseline that installers, homeowners, and researchers alike reference when estimating expected solar output.
At a glance: PVWatts is maintained by NREL, a US Department of Energy national laboratory, and its underlying methodology is also used, directly or as inspiration, in many commercial solar production estimate tools.
Why it matters
PVWatts matters because it gives homeowners a free, credible, independent way to sanity-check a production estimate from a solar installer's proposal. Rather than relying entirely on a company's own internal modeling, anyone can plug the same basic system parameters into PVWatts and get a comparable, independently sourced estimate to check the proposal's numbers against.
It's important to understand what PVWatts does and doesn't capture, though. Its inputs are relatively general: location, basic orientation, tilt, and system size, without detailed, site-specific shading analysis from a specific tree or chimney on your actual roof. NREL's own PVWatts documentation is direct about this limitation, noting the tool's predictions include inherent assumptions and uncertainties and don't reflect site-specific characteristics beyond what its basic inputs capture. A professional site assessment that accounts for real, observed shading conditions will generally be more precise for your specific property than a basic PVWatts run alone.
PVWatts also illustrates something worth understanding about any production estimate: it's built from historical weather patterns, not a prediction of exactly what will happen in any specific future year. Actual production in any given year will vary somewhat from the estimate due to real weather variability, which is completely normal and doesn't mean the estimate itself was wrong. To see PVWatts in a real example, the roof space vs output estimator in our System Sizing section applies it to a working system.
Where you'll see this
The annual kWh output estimator uses similar underlying methodology to PVWatts to project your system's expected yearly production.
Since PVWatts relies heavily on location-based solar resource data, the peak sun hours calculator shows the same kind of location-specific data PVWatts draws from.
This term comes up when homeowners research independent ways to verify an installer's production estimate, in academic and industry discussions of solar modeling methodology, and any time a production figure's underlying data source is worth understanding.
Related terms
Frequently asked
Yes, PVWatts is a free, publicly available tool provided by NREL, a US government research laboratory, and doesn't require payment or a subscription to use for basic production estimates.
PVWatts uses solid, well-established modeling based on decades of historical weather data, making it a genuinely reliable baseline estimate. Actual year-to-year production will still vary from any estimate due to real weather variability, since no tool can predict specific future weather with certainty.
Basic PVWatts inputs don't capture detailed, site-specific shading from a particular tree or nearby structure. It works from general location, orientation, and tilt inputs, so a professional site assessment with real shading analysis provides more precision than a basic PVWatts estimate alone.
Many do use PVWatts or PVWatts-derived methodology as part of their production estimates, sometimes combined with more detailed, site-specific shading and equipment modeling on top of the baseline PVWatts calculation.