NASA POWER
SolarVsGrid uses NASA POWER climatology data for global horizontal, direct normal and diffuse horizontal irradiance. These are modeled historical averages and should not be treated as a forecast for a particular day or year.
PVGIS
Where available, PVGIS is used to estimate photovoltaic output from location, array tilt and assumed system losses. The result is an energy estimate, not a guaranteed yield.
Important limitations
Local shading, roof orientation, temperature, equipment mismatch, clipping, soiling, downtime and weather variation can materially change actual production. A qualified installer should verify final design assumptions.