This paper proposes a method for detecting of small underperformances in the production of a WTG, using its power curve and control charts. As mentioned above, a change in the behavior of the WTG will results in a change in the power curve. When used along with control charts, power curve monitoring can be a resourceful tool for operators. Control charts, used in Statistical Process Control (SPC), monitor processes in real time and can raise the alarm when a process goes out of control, i.e. when a process is statistically different from a fault-free reference. Here we will study the use of Exponentially Weighted Moving Average (EWMA) and the Generally Weighted Moving Average (GWMA).