How WattGro calculates, and how we know it is right.

Irradiance comes from satellite-measured data for the cities in our list and from a modelled climate elsewhere; the result always says which it used. Sunlight is transposed onto the plane of your roof with the standard models, then reduced for temperature, soiling (higher in dusty climates), wiring, inverter and shading losses. Strings are checked at the design’s low and high temperatures, set per project; currents against IEC 62548 and NEC 690.8; row spacing against the winter solstice shadow.

We test the engine against PVGIS and against measured production from 12 operating plants.

The public estimator carries a band of about plus or minus fifteen per cent because it has not seen your roof. A full design, after survey, narrows that to about plus or minus five per cent.