The National Renewable Energy Laboratory has updated its PVWatts calculator, an interactive program that helps solar installers and homeowners estimate the performance of hypothetical grid-tied photovoltaic systems.

PVWatts creates hour-by-hour simulations and provides estimates for monthly and annual energy production, NREL says. A user can use default values that are built in or plug in values of their own, including the location and size of the PV array, the type of array, the tilt angle and the aximuth angle.

PVWatts is available in two versions. With the Site Specific Data Calculator, users choose a location from a map or list of locations around the world. With the Grid Data Calculator, users select any location in the U.S. from a 40-kilometer gridded map.

PVWatts originally went online in 1999 and has been updated a number of times since then. The newest beta version was released this January. NREL said it would keep PVWatts V1, PVWatts V2, and the PVWatts Viewer going until this summer while tweaking the new system. The program called IMBY (for In My Backyard), however, will be phased out in the first quarter of the year.