The validity of an isotropic model for predicting daily irradiation on inclined surfaces was tested by using one-year measured data on a horizontal, an inclined and four vertical surfaces for Easthampstead (51.4°N, 0.8°W) and Lerwick (60.2°N, 1.2°W) in the United Kingdom. As a consequence an anisotropic model has been developed. The proposed model treats south-, north- and east/west-facing surfaces differently. The accuracy of the proposed model is compared with that of the isotropic model by employing a simple technique of error analysis. The usefulness of a good daily inclined surface irradiation model is outlined.
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