The use of efficiency metrics for wind turbines is important for evaluating their productivity and quantifying the effectiveness of actions that are meant to improve their energy production. The recommends using annual energy production, the power curve, or the power coefficient, for the purpose of performance evaluation of wind turbines. This chapter describes three efficiency metrics commonly used for wind power production—availability, power generation ratio, and power coefficient. The availability tracks the amount of time in which power is produced by a turbine and then compares it to the amount of time when the turbine could have produced power. The revision is to use a power curve to provide the value of potential energy production under a given wind or weather condition. The capacity factor of a turbine is the ratio of the observed power output over the turbine’s maximum production capacity at the rated power.