Wind turbines start operating at wind speeds of 4 to 5 metres per second and reach maximum power output at around 15 metres/second. At very high wind speeds, i.e. gale force winds, (25 metres/second) wind turbines shut down. A modern wind turbine produces electricity 70-85% of the time, but it generates different outputs depending on the wind speed.
Over the course of a year, it will typically generate 15-30% (or more) of the theoretical maximum output of the turbine. This is known as its capacity factor, and modern turbines are getting reasonably close to theoretical limit of power one can extract from a stream of moving air, hence they are very efficient. However, the capacity factor is less than conventional power generators, but there are no fuel costs – the wind is free. So it doesn’t make sense to compare the ‘efficiency’ of thermal power systems with those of wind or solar or other renewable energy technologies.