Days when wind farms run at 10% capacity: Union say figures show renewable energy cannot be relied upon

• Wind turbines produced just 10 per cent of their energy capacity
• Monitors found 12 days when output dropped to 10 per cent of capacity
• Britain has heavily invested £1.25 billion in wind power energy  

Wind turbines produced just 10 per cent of their energy capacity during almost a fortnight of the last three months, it was claimed yesterday. 

Monitors tracking the energy generated from Britain's wind farms found 12 days when output dropped to 10 per cent of capacity or less, according to the GMB union.

It said its 'wind watch' figures demonstrated that Britain could not rely on renewable energy and needed nuclear or gas-powered plants to ensure its supply.

Britain has invested £1.25billion in wind power, which is now the country's biggest renewable energy source.

But critics have accused the Government and the National Grid of complacency over the risk of blackouts following the closure of coal-fired power stations.

A wind shortage last month (November) forced the National Grid to use new 'last resort' measures to keep the lights on in homes across the country on November 4.

Major industries were asked to down tools to protect energy supplies following high demand, power plant breakdowns and low wind power output.

At one point, wind farms were meeting only 0.5 per cent of the nation's electricity demand, compared to the average 10 per cent.

But in late November, dozens of wind turbines had to be switched off due to safety concerns when Storm Barney hit Britain.

Gusts of up to 85mph swept across the country, prompting fears they could overload the system or damage turbines.

The GMB, which supports more gas-fired power plants, said wind power produced 10 per cent or less of its energy capacity on 12 days during the three months from October 1 to December 21.

The figures related to wind farms connected to the national transmission system and not to turbines connected to local networks.

GMB General Secretary Paul Kenny said: 'The renewables lobby has to face up to the need for a base load electricity capacity that is reliable and clean on the days that the wind does not blow and the sun doesn't shine.

'When your electricity supply has 'Gone with the Wind', the response of the renewable energy suppliers that 'Frankly my dear we don't give a damn' is just not acceptable.'

Industry body RenewableUK has insisted that wind power is a 'success story' for Britain, and generated 9.5 per cent of the UK's electricity from July to September, the last period for which figures were available.

Overall, 23.5 per cent of the UK's electricity for the same period was generated by renewable sources, including bioenergy, solar and hydro power.

RenewableUK and the Department of Energy and Climate Change did not respond to requests for comment on the GMB figures.

A National Grid spokeswoman said: 'A diverse mix of generation is essential to the national transmission network in terms of security of supply.'