Electricity cannot be stored in large quantities, so electricity entering the system is moved around the country for immediate use. In Great Britain, this national electricity supply is maintained by National Grid in its role as the National Electricity Transmission System Operator (NETSO). A key function of the NETSO is to constantly ‘balance’ the supply and demand across the system. This is a process of always ensuring that the grid has sufficient electricity at any moment it is needed, which can vary greatly due to changing domestic and industrial demand and variable supply due to power station faults or changing wind conditions for wind energy generation.
Most consumers are not connected to the transmission system. As such, electrical energy is transferred from the transmission system to local distribution networks. These local networks run at lower voltages (up to and including 132kV in England & Wales and below 132kV in Scotland). Electricity from distribution networks is in turn transformed at a neighbourhood level to 400V (i.e. 240V in single phase) for domestic consumption.