A simple method for sizing phase-change energy storage (PCES) units for air-based solar heating systems is presented. An effective heat capacity for the phase change unit is obtained as a function of its mass, latent heat, specific heat, and melting temperature. The effective heat capacity can then be used, along with any convenient design method for systems with sensible heat stores, such as the f-chart method, to estimate the thermal performance of the system utilizing PCES. © 1979.