Generalized one- and two-time-constant models used to capture the evolution of the capacity of geothermal heat pumps at compressor start-up are presented. A one-time-constant model for the capacity decay at compressor shutdown is also presented. The determination of these time constants is important as longer time constants lead to more degradation of the heat pump performance in cycling conditions. Various reference time constants are obtained from a detailed and experimentally validated dynamic model of a geothermal heat pump in a range of operating conditions in heating or cooling mode. These dynamic empirical models are efficient and are more appropriate to whole-building energy simulation than corresponding deterministic models. It is shown that, contrary to the impression that may come from the literature, time constants change according to operating conditions. In particular, if the runtime decreases, the time constants tend to increase in the heating mode and to decrease in the cooling mode. They increase with the inlet water temperature (IWT) and decrease if the pressures are allowed to equalize at compressor shutdown. Finally, the time constants tend to increase when the fan is continuously operating. A time-constant-based heat pump model is derived and implemented in the dynamic energy simulation software TRNSYS.