with ρ the air density; A the area swept by the rotor; and cp the power coefficient of the WTG.
However, the shape of a WTG real power curve, as shown on Fig. 1, does not exactly follow this relationship. For wind speeds that are below the cut-in value (Zone I on Fig. 1), there is no power produced, since the wind does not have enough energy to move the rotor. At wind speeds above the nominal speed (Zone III), the power reaches its nominal value. Mechanisms such as pitch angle attenuation for active control turbines are used in order to maintain power at its nominal value. At extreme wind speeds, the WTG is stopped in order to ensure the structural integrity of the WTG (Zone IV). It is only between the cut-in speed and the nominal speed that a cubic relationship is observed (Zone II).