For sizing air-conditioning equipment, maximum solar heat gain factors (SHGFs) are used for estimating the design peak load, Recently, SHGF data representing the prevailing weather conditions have become important and practical for part load performance design. We have developed an approach for calculating SHGFs for the horizontal and vertical surfaces at the peak and other significant levels based on solar angles and clearness index (K-t). An analysis of the SHGF data has shown that the proposed model performs better than the ASHRAE clear sky approach, particularly at high significant levels (ASHRAE, American Society of Heating, Refrigerating and Air Conditioning Engineers). The findings provide information for the determination of total air conditioning plant capacity at both the peak load operation and the multiple equipment sizing under part load condition. This has implications for the management of energy use in the commercial sector as air conditioning is a major energy end user in commercial premises in subtropical climates. (C) 2000 Elsevier Science Ltd. All rights reserved.