Calibrating a wind vane of a wind turbine

A method of determining an offset angle to the wind direction measured from a wind vane of a wind turbine, the offset angle being used by a wind turbine controller to optimise power output of the wind turbine, the method comprising the steps of:

• determine speed of the wind preferably by means of an anemometer,
• determine wind direction preferably by means of the wind vane,
• the method is characterized in that it further comprises the steps of:
• defining a plurality of power bins representing an interval of power which can be produced by the wind turbine,
• determine an efficiency of the wind turbine for a plurality of time slots,
• determine the power output of the wind turbine for the plurality of time slots,
• comparing the efficiency of the wind turbine in two different time slots to obtain a highest efficiency, and
• updating a value of the power bin representing the interval of output power determined for one of the compared time slots, wherein the value of the power bin is updated with the result of the highest efficiency or a value derived from the highest efficiency multiplied with a constant.