A unit for installation in a complex product comprising an electric machine requiring an AC power supply. The unit comprises: a housing carrying an AC output; a battery in the housing comprising at least one battery cell; an inverter in the housing, the inverter comprising a plurality of voltage controlled impedances, VCIs, for providing a power supply to the AC output based on energy from the battery; wherein the housing carries a timing signal input configured to receive a timing signal from outside the housing; and wherein the timing signal input is coupled to control the VCIs so that changes in the impedances of the VCIs are synchronised with the timing signal.