The heating energy requirements for houses may be estimated by a simple calculation using the concept of seasonal solar utilization factors derived from hour-by-hour computer simulation of actual weather data for five Canadian locations. These utilization factors are presented for various levels of thermal storage as a function of the ratio of seasonal solar gain to seasonal building load. Curves are presented for conditions of constant room temperature and for temperature swings of 2.75°C and 5.5°C. Experimental data obtained from the Passive Solar Test Facility are shown to agree with calculations using the utilization factors.