Critics of wind farms have seized on the revelation as evidence of the unsuitability of turbines to meet the UK's energy needs in the future. They claim that the 'intermittent' nature of wind makes such farms unreliable providers of electricity.
The National Grid fears that on breezy summer nights, wind farms could actually cause a surge in the electricity supply which is not met by demand from businesses and households.
The electricity cannot be stored, so one solution – known as the 'balancing mechanism' – is to switch off or reduce the power supplied.
The system is already used to reduce supply from coal and gas-fired power stations when there is low demand. But shutting down wind farms is likely to cost the National grid – and ultimately consumers – far more. When wind turbines are turned off, owners are being deprived not only of money for the electricity they would have generated but also lucrative 'green' subsidies for that electricity.
The first successful test shut down of wind farms took place three weeks ago. Scottish Power received £13,000 for closing down two farms for a little over an hour on 30 May at about five in the morning.