Typically wind turbines start to generate electricity once the wind reaches 10 miles per hour, they reach maximum power output at around 33 miles per hour, but at very high wind speeds over 50 miles per hour, wind turbines shut down. This means power is generated at inconsistent levels.
Compared to many years ago, the efficiency of wind turbines has greatly increased, however claims from the industry that wind turbines run at an average of 30% capacity over a year, could be exaggerated by up to 25% according to The John Muir Trust (JMT), one of Scotland's leading conservation bodies.
A study carried out for the Trust between November 2009 and 2013 into the energy generated by dozens of wind farms, the majority of which are in Scotland, found they actually ran at 22% of capacity.
"This analysis shows that over the course of a year, the average load factor fell well short of what the industry claims, yet the 30% figure is peddled at every public inquiry into a proposed wind farm," said Helen McDade, head of policy at the JMT, "This data is needed to counter that hype".
The JMT said hundreds of wind farms had secured planning permission across Scotland based on inaccurate assumptions of their output.
The JMT examined the performance of 47 wind farms capable of producing 2,430 megawatts (MW) of green energy. They include Whitelee wind farm, near Glasgow, which comprises 322 turbines, and the 164-turbine Crystal Rig 2 development in East Lothian. Apart from Burbo Bank, Barrow and Thanet, which are offshore sites south of the border, all of the wind farms are in Scotland.
The research found over 395 days, the wind farms could have produced 17,586,000 MW hours of energy running at full capacity. In reality, 3,881,900MW hours was generated, equivalent to 22.07%.
And over the past two years, wind generation across the sites fell below 20MW on 123 separate days for a combined duration of 25 days. For a total of nine days, output dipped below 10MW, barely enough power to boil 3,300 household kettles.
"This is not a snapshot of wind energy output, it is a robust appraisal over a significant period of time," said Stuart Young, who was commissioned to investigate wind power by JMT. "It doesn't matter how many wind farms you build, if the wind isn't blowing, the blades aren't turning."
Niall Stuart, chief executive of Scottish Renewables, which represents renewable energy companies in Scotland, said the winter of 2009/10 was one of the calmest on record and that it was "no surprise" output figures for the year were below average.
He said, "It's well understood that there are variations within and between years, but no form of electricity works at 100% capacity 100% of the time."
These figures mean there is now an element of doubt about the validity of figures from the wind turbine industry and a lot more research is required to prove the point one way or the other.