An energy management system comprises a central processing unit for monitoring and controlling an electrical power supply, and one or more electrical devices capable of being plugged into electrical power sockets of that power supply using electrical plugs fitted thereto for providing that connection. The invention is concerned exclusively with square 3-pin plugs of the kind used in the United Kingdom. Each plug carries along the top edge of its earth pin, being the edge remote from the live and neutral pins, encoded data for identifying the associated electrical device, and each socket includes a reader positioned to read the encoded data when the plug is inserted into the socket, and means for communicating the encoded data to the central processing unit. The reader therefore communicates to the central processing unit in real time the identity of each electrical device connected to the power supply, which permits active control of the power supply.