Wind turbines are extraordinarily unreliable because wind speeds are so inconsistent; the average output of a turbine is only about a quarter of its capacity (know as the 'load factor'). Also, due to the vagaries of the wind, the turbines are unpredictable.
One question to ask is—what if the wind slows down, or ceases to blow at all? One example—in January 2010, Britain's wind turbines supplied virtually no power on most days. (1)
