One method of generating short, high‐power microwave pulses is to store rf energy in a resonant cavity over a relatively long fill time and extract it rapidly. With this pulse compression technique power gain roughly equal to the ratio of fill time to extraction time can be obtained. During the filling of a resonant cavity some of the energy is lost in heating the cavity walls, and some will generally be reflected at the input coupling of the cavity. In this paper, we discuss the time dependence of the stored energy and related quantities and the way in which it depends on the coupling of the source to the cavity.