A search for the best relation to be used to estimate monthly mean daily diffuse solar radiation on a horizontal surface for Europe

A number of relations are available to estimate monthly mean diffuse solar radiation on an horizontal surface (D) when the monthly mean daily global radiation (G) and /or the monthly mean daily number of sunshine hours (S) are known. In the present work, taking as a reference 264 values of D computed for the period 1966–1975 with the European Community solar radiation model, it is shown that many new types of relations can be found, which perform statistically better than those available. Mean monthly solar declination is a new, easy to use, variable to be taken into account when D has to be predicted over large areas with the highest possible accuracy.