SCOTLAND'S onshore wind farms were idle for record periods last year because of unusually calm weather, which industry analysts claimed could lead to higher power bills.
Turbines operated on average for just 21.9 per cent of the time - more than five percentage points less than in 2009, the Renewable Energy Foundation claimed. They are expected to operate at an average output of about 30 per cent of maximum capacity.
The group, which has warned of over-reliance on the power source, said onshore wind farms also produced the least electricity during cold weather, when demand was greatest. It said another factor in the cut in average output was wind farms being developed in less windy places because the windiest spots had been taken.
The foundation's comments followed UK government figures, published on Thursday, which showed UK onshore wind farms generated 7.7 per cent less electricity last year than in 2009, the first drop after years of steady increases. They contributed just over one quarter of the UK's electricity from renewable sources and less than 2 per cent of the total generated.
The figures also showed that offshore wind farms increased output by nearly 75 per cent last year as new turbines boosted capacity by nearly half.
However, hydro power contributed almost one third less electricity than in 2009 because of low rainfall. Hydro accounted for 14 per cent of renewable energy last year, slightly more than offshore wind farms.
Dr Lee Moroney, the Renewable Energy Foundation's planning director, said: "Experience is teaching us that wind power is not only highly variable over short timescales, but also from year to year and even in regions which have previously performed well.
"This finding has important economic implications for the conventional generators acting in the support role for wind. These face radical uncertainty about income from one year to the next. Unfortunately, the result is likely to be higher prices for consumers."
However, Beth Stratford, energy campaigner for Friends of the Earth Scotland, said: "The important thing is to put this in perspective. The possibility of wind output varying by 7-8 per cent from year to year is rather less worrying than the possibility of an enormous power station like Sizewell B having to shut down in an emergency and not reopen for six months, as it did last year.
"In terms of resilience and reliability, a decentralised energy system based on a broad variety of renewables wins hands down over a centralised system which relies on a handful of enormous power stations, which could fail with no warning whatsoever."
WWF Scotland head of policy Dr Dan Barlow said: "Data for the most recent quarter reveal a record contribution of electricity to the UK grid from wind, with enough electricity for over two million homes. As part of a renewable energy mix, wind energy has a very important role to play in helping us move away from fossil fuels and nuclear."
Niall Stuart, chief executive of trade body Scottish Renewables, said: "By definition, annual outputs will at times exceed the average and at times be below."