NREL is now the National Laboratory of the Rockies (NLR), and its tools have moved to nlr.gov. The links on this page point to the new addresses.
PVWatts is NLR’s free production calculator. You give it a location, a system size, a module type, an array tilt and azimuth, and a loss percentage, and it returns estimated monthly and annual energy production for a grid-connected PV system. It works anywhere in the world.
Version 8 runs on weather data from NLR’s National Solar Radiation Database, specifically the PSM V3 TMY 2020 dataset, and adds bifacial module options along with monthly irradiance loss inputs so you can account for soiling or snow month by month. Estimates from V8 will not always match older versions, which matters if you are comparing a new run against a number someone produced years ago.
How it gets used on a real project
PVWatts is the first-pass production number. It is what a developer runs before spending money, what a salesperson quotes in an early conversation, and what a lender will accept for screening but never for financing. Once a project is real, production moves to PVsyst or SAM with site-specific shading, a measured or purchased weather file, and a bankable loss stack. Treat PVWatts as the sanity check, not the answer: if your detailed model lands far from a PVWatts run for the same site, one of them is wrong and it is worth finding out which.