A major change between the wind turbine power performance testing standards of version 2017 and version 2005 was the definition of the wind speed. In version 2017, the wind speed used for power performance evaluation was the rotor equivalent wind speed (REWS). The definition of REWS was studied, and a simplified algorithm of calculating the blade lowertip section area and the blade uppertip section area was proposed. The validity and feasibility of two REWS calculations were provided via examples.