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Power curve (power generation versus wind speed) for two Vestas V90 models. They are most efficient at wind speeds of ~11–12 m/s (25–27 mph). At wind speeds of 25 m/s and above (≥56 mph), the wind turbines shut down.

Efficiency is often – incorrectly – confused with capacity factor.

Thermal plants (e.g., nuclear and coal) are rather inefficient, with around two-thirds of the energy released from their fuel being lost to heat. But they can have a very high capacity factor, particularly if they are used to provide base load.

Wind turbines, on the other hand, can be quite efficient, capturing and converting most of the energy from the wind that is physically possible (the Betz limit), although only within a rather narrow range of wind speed.

At wind speeds slower than ideal, there is not enough wind energy to generate power at the full rate of the wind turbine’s design.

At faster wind speeds, the blades must be pitched so that their rotation rate remains constant, thus reducing efficiency.

The result is that wind turbines have a rather low capacity factor of 25%–35%, and that is due to the intermittent and variable wind instead of the demands of the grid.

See also: Availability.