The validity of various models for predicting irradiance on inclined surfaces was investigated by employing one-year measured hourly irradiation data on horizontal and vertical surfaces for Easthampstead (51.4°N, 0.8°W) in the United Kingdom. This led to the development of an improved anisotropic model that distinguished between shaded and sunlit surfaces and between overcast and non-overcast conditions of the sunlit surfaces. The accuracy of the proposed model was compared with those of Klucher and Hay and the model was further tested against short-term measured data for four widely separated locations in the United Kingdom.
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