The fraction of the kinetic energy of the wind impinging on its area, that a wind turbine can convert to useful power, has been shown by Betz in an idealized laminar‐flow model to have an upper limit of 16/27 or 59%. The limit is here simply rederived and it is shown how deviations from the idealized model, involving rotational kinetic energy of the downwind stream and turbulent mixing from outside the boundaries of the idealized stream, can either increase or decrease the power available. The limit is thus not a strict upper limit in practice.