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⎯ The main objective of this study is to review and test the applicability of wellestablished models collected from the literature for estimating the monthly average daily diffuse solar radiation on a horizontal surface in Egypt. The different meteorological data measured at eight stations during the period 1987–2016 were used to calculate the monthly mean values of diffuse solar radiation over these stations using the collected models. The selected eight stations measure diffuse solar radiation component and have been chosen to cover the whole of Egypt. The collected models (fourteen models) were compared on the basis of many statistical error tests such as the relative percentage error, (e%), mean percentage error (MPE), mean bias error (MBD), root mean square error (RMSE), t-test, and Nash-Sutcliffe equation (NSE). According to the results, the Tarhan and Sarı model (Model 12) showed the best estimation of the diffuse solar radiation on a horizontal surface for all of the eight stations, and therefore, it is recommended for predicting diffuse solar radiation at any location in Egypt. Key-words: solar energy, diffuse solar radiation, sunshine duration, extraterrestrial radiation, solar radiation models, model comparison, Egypt.

Validation of the existing models for estimating diffuse solar radiation over Egypt