Two heat pumps were tested at several "on"-"off" cycles to determine performance at part load conditions. Also, the heat pumps were tested at full capacity under frost buildup conditions with both time-temperature and demand defrost controls. The objectives were to develop simulations for use in seasonal performance models. For "on"-"off" cycling and defrost cycles, it was found that COP as a fraction of steady state COP could be predicted with power law relationships involving capacity as a fraction of steady state capacity. Frost buildup occurred in a two-step process, with initial gradual frost buildup to a transition point followed by rapid frost buildup until a defrost cycle was initiated. Heat capacities under frost buildup conditions were predicted as a function of outdoor air temperature and difference between specific humidity of the outdoor air and specific humidity of saturated air at outdoor coil temperatures.