PURPOSE: To provide an emergency stand-by power supply in which individual generators can be operated under an appropriate load by supervising the load and the accumulated operating time of each engine generator constantly and controlling the number of generators to be operated and the starting sequence thereof.

CONSTITUTION: The inventive system comprises an emergency stand-by power supply for backing up a commercial power supply by operating a plurality of engine generators in parallel, and a supervisory controller for centrally supervising and controlling the emergency stand-by power supply. The supervisory controller 11 supervises the amount of load to be fed with power from the stand-by power supply and controls the number of generators to be operated such that each engine generator 1, 2, 3 is loaded appropriately. The start set value of each generator 1, 2, 3 is varied based a command from the supervisory controller 11 and the starting sequence of each engine generator is controlled every time when power is interrupted. The supervisory controller 11 supervises the accumulated operating time of each engine generator and controls the averaging of the operating time of each engine generator in combination with the number of generators to be operated.