The electricity grid is a dynamic entity in a constant state of flux as the levels of supply from generators and demand from all consumers constantly change. Electricity cannot yet be stored economically: excess supply one day cannot be used to meet a surge in demand on another day. So the grid’s operators must ensure that the amount of available electricity matches the amount consumed at any time, and vice versa. The grid must be able to respond to predictable movements in demand patterns as well as random, second-by-second changes. The grid’s operators must also have plans in place to manage a major interruption to supply, such as a power station failing. To keep the grid balanced, traditional generators have a throttle control that enables them to vary their output instantaneously – this is known as response. The grid also maintains a buffer of spare capacity from ‘balancing stations’ which can be called on at extremely short notice. If a power station goes down, or there is a problem in transmission, these ‘spare’ stations swing into action. The problem the grid operators have is that this insurance policy is expensive. It adds to the cost of power supply and, because it must be kept running at a low level at all times, it produces CO2 emissions even when power is not being used.