This means that the amount of time a wind turbine produces a given power output is just as important as the level of power output itself. Expected energy output per year can be reliably calculated when the wind turbine's capacity factor at a given average annual wind speed is known. The capacity factor is simply the wind turbine's actual energy output for the year divided by the energy output if the machine operated at its rated power output for the entire year. A reasonable capacity factor would be 0.25 to 0.30. A very good capacity factor would be 0.40.
Capacity factor is very sensitive to the average wind speed.
When using the capacity factor to calculate estimated annual energy output, it is extremely important to know the capacity factor at the average wind speed of the intended site. Lacking a calculated capacity factor, the machine's power curve can actually provide a crude indication of the annual energy output of any wind turbine. Using the power curve, one can find the predicted power output at the average wind speed at the wind turbine site. Actually, because of the effect of the cubic power law , the annual energy output will probably be somewhat higher than this figure at most windy sites. This is determined by the wind power distribution, which shows the percentage of time the wind blows at various wind speeds over the course of an average year.