Not to state the obvious, but in order to generate electricity from a wind turbine you need to have wind. But, what may not be obvious is that wind turbines can only operate in a range of winds. If the wind speed is too low (below the “cut-in” speed) or too high (above the “cut-out” speed), the turbine will not produce any energy. This is illustrated in the following diagram:
The amount of time that a turbine can generate electricity is dependent on the distribution of wind speeds. That is, how often does the wind blow at a given speed at the location where the turbines are located. This distribution is typically what is called a Weibull Distribution and must be determined by actual wind measurements at the proposed installation site for at least a year. A typical graph of wind speeds at a location is shown below:
A website that has a lot more detail and useful information about wind speeds, turbine efficiency, etc., is http://www.wind-power-program.com/wind_statistics.htm
The following FAQ from the National Wind Watch is very helpful in explaining the various terms and aspects of wind turbine output: National Wind Watch Output From Industrial Wind Power