A power-consumption monitoring device, for taking measurements of the power consumed by an appliance. The device is connectable to a mains electrical circuit and operable to transmit the measurements to a master device through the circuit. Also presented is a master device, for receiving over a mains electrical circuit power-measurements transmitted by one or more power-consumption monitoring devices. Also presented is a power-line communications network comprising: a master device; and one or more power-consumption monitoring devices.