'The UK has more usable wind power than any other European country and large scale wind turbine farms on and offshore will become a major supplier of electricity to the national grid in the next twenty years.The BBC always gives us the maximum capacity of every wind-farm with its promise of free, clean electricity.
As at 28 June 2009 there are 217 UK grid-connected wind farms containing 2,537 wind turbines with the capacity to generate 3628 MW.*'
Wikipedia states that:
'As of 1 December 2010, there were 282 operational wind farms in the UK, with 3,149 turbines and 5,194 MW of installed capacity. A further 2,283 MW worth of schemes are currently under construction, while another 6,245 MW have planning consent and some 9,328 MW are in planning awaiting approval.Wikipedia do admit near the end of their article that:
'Because the level at which a wind farm operates is determined by the wind speed at any particular time, capacity factor bears limited relation to the hours it operates. In comparison, thermal power stations must run at or near their full capacities at all times in order to achieve maximum efficiency. This makes them ill-suited to act as a supplement to wind power. An additional problem is that when winds are outside the optimal range for wind turbines (5 to 25 m/s), they cannot generate any power. If this happens during a winter cold snap, when electrical demand reaches its highest levels of the year, conventional power sources must have the capacity of meeting that entire demand.  
In 2007, UK wind farms achieved "load factors on an unchanged configuration basis" of 27.3% for onshore and 28.3% for offshore respectively. In 2008 onshore farms achieved 29.4% and offshore farms achieved 34.9% on the same basis. A study by the Renewable Energy Foundation (REF), which campaigns against the use of wind energy in the UK, claimed that only a few Scottish wind farms achieved the 28% average, while turbines in lowland England were operating at much lower levels, some at less than 10% of capacity.'
Note those 27.3% and 28.3% figures because today I note that NETA reports that wind farms are producing not 3,628MW or more than 5GW or even 27.3% of 5GW but forecast today of 373MW and tomorrow of 46MW. Why are we spending so much money on a power generating source that is so inefficient? By way of comparison the The coal-fired Ratcliffe-on-Soar Power Station in the UK has a rated capacity of 2 GW and will run at or near that generating capacity for long periods of time; not just when the wind blows.
If you want more information as to the actual electricity generation by Wind Farms than I recommend a perusal of The Renewable Energy Foundation's figures.