Wind turbines are very efficent. However we do not rate wind turbines on efficency in the same way as conventional power stations. We have two measures to rate a wind turbne or wind farm.
The first is known as energy yield which is how much energy it will produce on a particular site.
The second term is known as capacity factor, this is a ration of the amount of time that the wind farm is operation at its maximum output. A goo wind farm will have a capacity factor of about 40% which means that forty percent of the time ists operating at its maximum utput.