System power control using a power supply to provide voltage to subsystems in which a subsystem determines its operating state based on the voltage received by the subsystem

A power control unit 16 controls the power consumed by a system 10 having a plurality of electrical power consumption states including an active state and at least one power saving state. The system comprises a plurality of subsystems 12 each having a plurality of subsystem power states including an active state and at least one power saving state. The electrical power consumption states of the system correspond to respective different configurations of subsystem power states of the subsystems and the power control unit is arranged for signal interface with the subsystems and a power supply unit 14 of the system for controlling the subsystem power states of each of the subsystems and the power supplied to each of the subsystems by the power supply unit dependent on the required power consumption state of the system.