Control of storage devices in an electric power network

A method of controlling the transfer of electricity between a collection of a plurality of electricity storage devices 110, having a communication line therebetween, and an electrical power network comprises receiving a message at a first electricity providing device 110A from a controlling device 310 and, in response to the message: transferring electricity between the first storage device and the power network; and sending, from the first storage device 11A to a second electricity storage device 110B, a second message containing data in response to which the second electricity providing device transfers electricity to or from the electrical power network. The controller 310 can be a server or a further storage device and receive an acknowledgement from the first device in response to the transfer of electricity and receipt of an acknowledgement form the second device. Storage devices can store an identifier that identifies the second storage device, which can be updated depending upon ...