Window solar heat gains contribute significantly to building heating and cooling loads. It has been important for some time to accurately estimate these loads on clear days for sizing cooling equipment. Recently, average day heat gains have also become important for estimating energy use. This paper develops linear relationships between the ratio of average to clear day Solar Heat Gain Factors (SHGF) and long-term Percent Possible Sunshine (PPS) data. Average-day SHGFs were estimated using long-term monthly average, daily, horizontal solar radiation data for five U.S. cities. Linear correlations were obtained for five different window orientations: south, southeast/southwest, east/west, northeast/northwest, and north. The results show that the common assumption of direct porportionality of average to clear day SHGF with respect to PPS is not very good for northerly window orientations, but it may be adequate for other orientations.

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