Abstract: A diesel powered emergency electrical generator for providing a back-up power supply for critical building systems. The diesel engine is heated by a heat pump in series with a resistance heater. The heat pump is normally cycled on and off to maintain the desired engine temperature and the resistance heater is employed only when the heat pump is unable, for whatever reason, to maintain the desired engine temperature. The heat pump includes a refrigerant to coolant condenser and an air to refrigerant evaporator and includes a centrifugal pump which operates continuously to maintain a continuous circulation of coolant through the engine and through the heat pump.