Global and diffuse solar radiation intensities are, in general, measured on horizontal surfaces, whereas stationary solar conversion systems (both flat plate solar collector and PV) are tilted towards the Sun in order to maximize the amount of solar radiation incident on the collector surface. Consequently, the solar radiation incident oil a tilted surface must be determined by converting the solar radiation intensities measured oil a horizontal surface to that incident on the tilted surface of interest. There exist a large number of models designed to perform such a conversion. 11 Such models have been tested utilizing data measured in Beer Sheva, Israel. The data consist of hourly global and diffuse solar radiation oil a horizontal surface, normal incidence beam and global radiation oil a south-oriented surface tilted at 40 degrees. The horizontal diffuse radiation measured using a shadow ring was corrected using four different correction models. This resulted in 44 model permutations, The individual model performance is assessed by an inter-comparison between the calculated and measured solar global radiation oil the south-oriented surface tilted at 40 degrees using both graphical and statistical methods. The relative performance of the different models under different sky conditions has been studied. Different grading systems have been applied in an attempt to score the relative performance of the models. (c) 2008 Elsevier Ltd. All rights reserved.