Wind farms are an excellent secondary power source. They cannot however be primary, as a high pressure day wipes them out... Offshore make more sense to me where possible, but there you go.

The efficiency is another issue. The carbon footprint of manufacturing and installation mean a rather long time before they pay back their carbon footprint, allied to ongoing maintenance and possible service life.

It was said above that efficiency is a wide ranging term. In energy calculations it remains less than 100% at all times regardless as the cost of wind is still a cost, mathematically speaking. If you look at it otherwise then everything is fully efficient as you can dig backwards till you reach the energy of the sun anyway.