"During each of the four highest peak demands of 2010, wind output reached just 4.72%, 5.51%, 2.59% and 2.51% of capacity"

BBC News - Wind farm efficiency queried by John Muir Trust study

Wind farms are much less efficient than claimed, producing below 10% of capacity for more than a third of the time, according to a new report.

The analysis also suggested output was low during the times of highest demand.
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But the research found wind generation was below 20% of capacity more than half the time and below 10% of capacity over one third of the time.
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During each of the four highest peak demands of 2010, wind output reached just 4.72%, 5.51%, 2.59% and 2.51% of capacity, according to the analysis.