This summary covers the principal findings of an analysis of electricity generation from all the UK wind turbine farms which are metered by National Grid, from January 2011 to December 2012.
The analysis shows:
- The total output of the monitored wind turbines (as measured by the National Grid) increased from 2,430MW to 5,705MW over the period
- The average capacity factor for all monitored wind turbines, onshore and offshore, across the whole of the UK was 33.2% and 30.7% in 2011 and 2012 respectively.
- The average capacity factor in any given month varied from 16.2% (July 2011) to 50.8% (December 2011) with 9 months having an average output of <25% of monitored capacity and 12 months <30%.
- The time during which the wind turbines produced <10% of their rated capacity totalled 3,165 hours or 131.9 days – 18% of the two year time span studied.
- The time during which the wind turbines produced <5% of their rated capacity totalled 1,200 hours or 50 days – almost 7% of the two year period.
- The output from wind turbines is extremely intermittent, with output varying by a factor of 10 over very short periods.
Three assertions are commonly used to support industrial wind turbine installations:
- Wind turbines generate on average 30% of their rated capacity over a year
- The wind is always blowing somewhere in the UK
- Periods of low wind across the UK are infrequent
However, the conclusions to be drawn from the evidence are:
- In windy years such as 2011 and 2012 turbines can, on average, produce over 30% of their rated capacity, but this is certainly not the case every year.
- The assumption that the wind is blowing somewhere in the UK at any given time is, in practical terms, false: there are regular periods when there is not enough wind to contribute to any meaningful power generation.
- Periods of low wind are so frequent that wind turbines cannot be relied on as a steady source of power, even given two-fold increase in installed capacity over the period studied. Wind turbines must be backed up by the equivalent capacity of conventional fossil-fired power stations, thus largely negating any fuel savings or reductions in CO₂ emissions.