This paper presents a methodology to aid a system or building designer in the quick sizing of a Direct Gain passive solar system with night insulation. The method developed and discussed here will approximately predict annual energy savings for a range of passive system sizes based on annual horizontal solar radiation, latitude, and the estimated annual heat loss of the building without passive. The design aid format is a series of graphs calculated for several climates in the U.S. This design aid will help in quickly evaluating design changes for system performance and financial benefit.