An electricity supply that includes variable energy sources can still be reliable overall, provided there is sufficient diversity in the generating mix. Flexible energy sources that can alter their electrical output quickly can be called on when the less predictable intermittent energy sources fail to supply.
Some coal-fired and gas-fired power stations and most hydroelectric ones can alter their output rapidly enough to fulfil this function. The next generation of nuclear power stations could be able to do so as well. However, building additional plant to be ready as backup could raise the overall cost of electricity.
It is therefore important to have a secure level of baseload power – the minimum electricity supply required to keep the lights on. This is also typically fulfilled by power stations that use gas, coal or nuclear fuel. These excel at producing a continuous, high electrical output.
If electricity could be efficiently stored, it could be used to bolster supply when there is a danger that demand might not be met. Pumped hydroelectric storage is one solution. During periods of low demand for electricity, water is pumped into an elevated reservoir, and when extra electricity is required, the water is allowed to flow downhill past a turbine generator. However, this solution is costly and there are not many suitable places to develop such storage in the UK. Researchers are looking at other ways to store electricity in bulk economically, including advanced battery technology.