The utilization factor, which is a function of weather and building thermal response, is the ratio of the utilized to the total solar-heat gain in a building. It is determined using data obtained from a weighted zone-by-zone dynamic hour-by-hour thermal analysis of solar-energy absorption in a generic office building. Useful solar-energy absorption in a building depends on a number of building parameters; it is especially dependent on zoning and time constant. This utilization factor, which in this study is a continuous function of time constant, is used in the Early Design Model (EDM) which is a simplified steady-state design tool to take account of dynamic heat flows within the building.