We demonstrate quantitatively that whether a geomagnetic storm results in an increase or decrease of relativistic electrons in the outer zone depends on the competition between the physical processes producing the energization and loss of electrons. We construct a simplified one-dimensional model for the electron energy distribution incorporating electron energization by cyclotron resonant interaction with whistler-mode chorus, and electron losses due to pitch angle scattering into the loss cone by combined plasma waves (in particular, electromagnetic ion cyclotron waves and plasmaspheric hiss). We do not include radial diffusion explicitly since the model applies to the region 3 < L < 5 where radial diffusion is expected to be weak. We treat the chorus wave amplitude and electron loss rate as model input variables and compute the solution for the electron energy distribution as the model output. The extent of the electron flux increase or decrease during the event is found to be sensitively controlled by the magnitudes of the wave amplitude and electron loss rate.