A demand side electric power supply management system is disclosed. The system comprises an islanded power system having a point of coupling to a supply grid. The islanded power system supplies a plurality of electric loads, each of which is associated with a load controller to control the maximum power demanded by that load. A measuring means associated with the point of coupling measures the total power transfer between the grid and the islanded system, and a system controller monitors the measured power transfer relative to a set point and provides a control signal to a plurality of load controllers. Each load controller receives substantially the same control signal and determines the maximum power which the or each load associated with the load controller is allowed to draw from the islanded power system based on information contained in the control signal.