An analytical method is developed for estimating R, the ratio of the long-term monthly-average daily radiation on an inclined surface to that on a horizontal surface. This method differs from the Liu and Jordan [1] method in the manner in which the beam radiation component is determined. The method is applicable for surfaces of any orientation. R values calculated in this manner and by the Liu and Jordan method are compared with integrated hourly calculations for 23 years in Madison, WI, Albuquerque, NM and Miami, FL; the comparisons show that the method described in this paper agrees more closely with the integrated hourly calculations, especially for surfaces facing east or west of south.

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