Abstract: A battery/mains generator set for the production of a continuing electricity supply in the event of failure of the mains supply, the set comprising an a.c. machine capable of operating either as a generator or as a motor and, mechanically coupled thereto, a d.c. machine also operable either as a generator or as a motor, the a.c. machine being, in operation, permanently connected, with the mains supply through a static switch, which prevents back-flow of power into the mains, the a.c. machine, when mains driven, being operable as a motor and driving the d.c. machine, which then operates as a generator and charges a battery in accordance with a required I/V characteristic curve, and, on failure of the mains supply, the d.c. machine being operable as a motor powered by the battery to drive the a.c.