1. A method of controlling access to an electrical power network, comprising: receiving a power network access rule from a power network, the power network access rule defining an access condition which permits a device to access the power network: receiving a device usage rule, the device usage rule being a condition when the device is permitted use of the power network; comparing the device usage rule with the received power network access rule; and permitting the transfer of energy between the power network and the device upon a positive comparison between the device usage rule and the power network access rule.