A method of operating a transport refrigeration system comprises: controlling, using a controller (30), a plurality of components of the refrigeration system and monitoring, using the controller, a plurality of operating parameters of the refrigeration system. The controlling comprises operating at least one of a prime mover (26), heater (48), and electric generation device (24). The operating parameters comprise at least one of a speed of the prime mover and a voltage of the electric generation device. The method comprises detecting, using the controller, when at least one of a heating mode and a defrost mode is required; activating, using the controller, the heater when at least one of the heating mode and the defrost mode is required; comparing, using the controller, the voltage of the electric generation device to a selected voltage; and controlling, using the controller, the speed of the prime mover in response to the voltage of the electric generation device.