A model for building heat loss that includes hourly variation in solar radiation and windspeed was designed for use in evaluating solar heating system performance. Numerical simulations were run to see if, from an economic standpoint, the two effects were sufficiently large that a different solar collector size would be chosen than that selected using a heat loss model incorporating temperature only. The results show that solar system performance for the two models is significantly different on a daily basis but similar on a long term basis due to compensating factors. Thus, the simple model is adequate for selecting the optimum size of a solar collector.