Introducing the best model for predicting the monthly mean global solar radiation over six major cities of Iran

In this study, by using long-term global solar radiation data and other meteorological parameters, 11 empirical models taken from the literature were tested for prediction of monthly mean daily global solar radiation over six major cities of Iran, named Isfahan, Karaj, Mashhad, Shiraz, Tabriz and Tehran. The models are from 3 categories: (1) Only function of sunshine duration; (2) Function of sunshine duration as well as relative humidity and ambient temperature; (3) Independent of sunshine duration and function of relative humidity, ambient temperature and its maximum and minimum. The models were established using statistical regression technique and their accuracies evaluated using the statistical indicators of mean percentage error (MPE), mean bias error (MBE) and root mean square error (RMSE); hence the best model in each category was recognized. Although, 5 out of the 6 nominated cities fell in climatic classification of BSk, it was not possible to introduce a single model even for the cities having the same climate type. However, for all of the cities the best model was either from categories (1) or (2). In line with increased tendency toward installation of solar systems in Iran, the proposed models are useful for simulations and incorporation in design procedures.