Effective accuracy of satellite-derived hourly irradiances

Estimates of hourly global irradiance based on geostationary satellite data with a resolution of several (2 to 10) kilometres reproduce ground-measured values with a Root Mean Square Error (RMSE) of typically 20% to 25%. The different components of this RMSE have been enumerated by several authors but, due to the lack of adequate measurements, their respective importance is not well settled. In the present study we attempt to quantify these components from a practical point of view, that is from the point of view of users having to rely on time/site specific irradiance data. We conclude that the intrinsic, or "effective" RMSE is more along the line of 12%. This effective RMSE is the measure of the methodological imprecision (satellite-to-irradiance conversion models). The remaining part of the overall RMSE is the amount by which spatially averaged satellite-derived estimates are, by their very nature, bound to differ from ground-measured local insolation.