This paper reports on the use of energy in single compartments in Hong Kong, calculated by the building performance simulation program BLAST 3.0. Effects of volume and geometrical shape of the compartments on the estimated annual energy requirements are studied. The program is suitable for use as an engineering design tool for energy estimations, including the selection of HVAC systems for a commercial building. In running the package, a climatic data-file compiled for a typical meteorological year in Hong Kong was used. Three types of rectangular compartment of volumes varying from 4000m(3) to 40000m(3) were considered. The use of energy in buildings with concrete roofs is compared with cases with glass roofs.