1. A method of controlling access to an electrical power network, comprising: receiving, at an energy access point, a power network access rule from a distribution operator via a power grid, the power network access rule defining an access condition which permits a device to access the power grid; receiving, at the energy access point, a device usage rule from the device, the device usage rule being a condition when the device is permitted use of the power grid, and the device usage rule being provided to and stored in the device from an energy provider during an initial registration of the device with the energy provider; comparing, at the energy access point, the device usage rule with the received power network access rule; and permitting, at the energy access point, transfer of energy between the power grid and the device upon a positive comparison between the device usage rule and the power network access rule, wherein the distribution operator provides a distribution network for the energy, and wherein the energy provider buys and sells the energy.