Calculates analytically the energy gained by an electron during one transit of an idealized one dimensional magnetic mirror field B in the presence of a given microwave electric field. The heating rate is defined as this energy gain over the transit time. It depends on the initial energy and the position of the turning point of the electron. The heating rate for relativistic electrons not crossing the resonance surfaces can be comparable to that of 'resonant electrons'. A comparison of the heating rate and energy loss rates from multiple collisions and radiation suggests an equilibrium at energies of 10 2-103 keV, depending on the mirror ratio, microwave field strength and frequency, and the density of the cold electron group responsible for the collisional energy losses. The fraction of electrons heated to this equilibrium energy is calculated.