Abstract
The total radiation data measured at Dhahran (26 degrees 18'N lat., 50 degrees 08'E long.), Saudi Arabia, on a surface inclined at 26 deg from the horizontal for the period March 1984-April 1985 were used to test an isotropic and two anisotropic models for calculation of total radiation on inclined surfaces. The total and diffuse radiation amounts measured on a horizontal surface were used when calculating with these models. The models were compared on the basis of the statistical error tests by using the root mean square error (r.m.s.e.) and the mean bias error (m.b.e.). The r.m.s.e. varied between 0.399 and 5.578%. The results were compared with similar calculation for Woodbridge, Ontario, Canada (43.8 degrees N lat.). The comparison showed that the choice of one or another model depends upon location, inclination of the surface, and time. For Woodbridge, the anisotropic models of Klucher and Hay were found to be more accurate than the isotropic model. This study shows that, for hot arid areas, the isotropic model is more accurate for tilt angle values around the latitude of the location.