Statistical comparison of correlations for estimation of global horizontal solar radiation

We evaluate the statistical performance of four correlations for estimating the monthly average daily horizontal global radiation. We have used bright sunshine and global radiation data for 140 locations around the world with varied meteorological conditions and geographical locations. The results show that Rietveld's equation generally gives the best estimates. Hay's correlation is next best, whereas the Glover-McCulloch relation gives the maximum error. The computations in this study are based on the most recently adopted solar constant.