If there is a grid outage cutting electricity supply to the ASHPs, there will be gas boilers in place as a back-up (the most cost-effective way of providing a backup at this point in time but our intention is to move away from the use of gas as early as economically possible). Additionally the BEN has 3 huge stores of hot water (thermal stores) which also supply heat to customers. This means there are three methods of supplying heat to customers.