The availability factor for a power plant has historically meant the probability that when you needed it to be working, it was ready.  For example, hydropower plants are often used for providing peak power, and may be used only 25% of the time.  However, if, in a given year, the power plant successfully produces power every time it was asked to do so, its availability factor is 100%.  For wind turbines, the same words are misleadingly used to describe something entirely different.  If a wind turbine is in good mechanical shape and produces power whenever the wind blows, then its availability factor is said to be 100%. The difference is this:  With normal power plants we ask whether power is available when it is wanted.  With wind turbines we ask whether power could be produced if the wind were blowing.