The present invention is a back-up power supply of alternating current and direct current, with the alternating current being generated by an inverter from the direct current, which is produced by a fuel cell. The inverter is at least partially encompassed by a phase changing material having an elevated melting point such that the phase changing material absorbs heat generated by the inverter as well as in the surrounding air, thereby maintaining the inverter temperature within acceptable limits. As the phase changing material absorbs the heat, its temperature rises and melts. Upon cooling of the system, the phase change material re-solidifies.