An analysis funded by conservation charity the John Muir Trust has found wind turbine claims are overstated and output was low during the times of highest demand, producing below 10% of capacity for more than a third of the time.

The research was carried out by Stuart Young Consulting and analyzed electricity generated from UK wind farms between November 2008 to December 2010.    The wind industry and government agencies claim wind turbines will generate on average 30% of their rated capacity over a year and stated they have no confidence in contrary data.   The report concluded turbines "cannot be relied upon" to produce significant levels of power generation.

Wind farm efficiency queried by John Muir Trust study - BBC