Abstract: A stand-by electrical energy source for equipment to be operated with substantially continuous current supply, comprising three components one of which is at least one electric rotary machine, another of which is a flywheel and the third of which is an auxiliary energy source. Planetary gearing is provided, comprising a central shaft on which is mounted a sun gear, a planet gear and a ring gear, each of the three components being connected with a respective one of these gears. Two of the three components are interconnected by a drive with an infinitely variable transmission. In one embodiment, the electric rotary machine is connected to the planet gear, the flywheel is connected to the sun gear and the auxiliary energy source is connected with the ring gear. In another embodiment, the electric rotary machine is connected to the sun gear, the flywheel is connected to the ring gear, and the auxiliary energy source is connected to the planet gear.