In subtropical Hong Kong, solar heat through fenestration, particularly on vertical surfaces, plays a major role in determining the thermal performance of a building. For sizing air-conditioning equipment, maximum solar heat gain factors (SHGFs) are used for estimating the design peak load. Recently, the SHGF data representing the prevailing weather conditions have become important and more practical for evaluating daylighting designs and the determination overall thermal transfer value (OTTV). This paper describes a procedure for calculating hourly SHGFs for horizontal and vertical surfaces based on the 8-year (1991 to 1998) horizontal global and diffuse radiation and the 3-year (1996 to 1998) vertical global solar radiation data measured at the City University of Hong Kong. A simple approach for estimation of solar heat gain is developed. Average and peak SHGFs and sky factors for OTTV are computed and compared. An analysis of the SHGF data has shown that except for the north-facing surface, the ASHRAE clear sky approach tends to overestimate the solar heat gains, particularly at large significant levels. The findings provide information for the determination of the total air-conditioning plant capacity at peak load operation and multiple equipment sizing under part load condition. (C) 2000 Elsevier Science S.A. All rights reserved.