Abstract: An uninterrupted power supply (UPS) system provides power to an electrical load when utility line power fails. When the utility line power is normal a synchronous dynamo serves as a motor, and a first variable-ratio transmission arrangement is combined with a differential gear box to drive a flywheel with very low losses. When the utility line power fails, further variable-ratio transmission components are activated to assist the first transmission in conducting power from the flywheel to the dynamo. The dynamo now acts as a generator to supply power to the load. The invention is characterized by high standby efficiency and by a capability to run the flywheel down to a very low speed before it is unable to continue driving the dynamo.