Abstract: A method and system for maintaining a substantially constant output of electrical power includes an engine for operating upon receipt of a signal, an electric motor, a flywheel mounted to rotate when either the engine or the motor is operated, a generator for producing electrical power when operated, and a hydraulic coupling system for driving the generator when the flywheel is rotated. A control unit allows application of power from a conventional power source to the electrical motor so long as the output power level from the conventional power source is adequate and thus the flywheel is rotated and the generator is driven to provide output power to a load. The control unit also allows application of power directly from the power source to the load. Upon interruption of power from the conventional power source, the control unit causes cut-off of power from the source to both the motor and the load, and applies a signal to the engine to start the engine.