Method of improving an accurateness for estimating an available power production of a wind turbine (900, the method comprising measuring a current power (Pa) being generated by the wind turbine, measuring a current rotor speed (ωa) of a rotor of the wind turbine, measuring a current blade pitch angle (θa) of a rotor blade of the rotor, defining a three dimensional reference look-up table comprising three orthogonal axes, wherein the first axis denotes a rotor speed (ω), the second axis denotes a wind speed (v) and the third axis denotes a blade pitch angle (θ), and wherein the values in the three dimensional reference look-up table denote a power being generatable by the wind turbine dependent on the wind speed (v), the rotor speed (ω) and the blade pitch angle (θ), determining the estimated wind speed (ve) based on the current power (Pa), the current rotor speed (ωa) and the current blade pitch angle (θa) by a processing unit which looks up the estimated wind speed (ve) in the reference look-up table for the current power (Pa), the current blade pitch angle (θa) and the current rotor speed (ωa), correcting the blade pitch angle (θ) in the reference look-up table, wherein the correcting comprises a calibration cycle, the calibration cycle comprising determining if the current power (Pa) is an available maximum power (Pmax) of the wind turbine based on the determined estimated wind speed (ve) and a power curve of the turbine, wherein the power curve indicates the available maximum power (Pmax) at different wind speeds (v), then derating the power production of the turbine until the wind turbine produces again a limited power (Pl), wherein the derating of the power production of the turbine to the limited power comprises derating the power production of the limited power by changing the current blade pitch angle (θa) and by keeping the rotor speed (ωa) constant, measuring the current blade pitch angle (θa) and calculating the calculated blade pitch angle (θt) at the limited power (Pl), wherein the calculated blade pitch angle (θt) is calculated by the limited power (Pl), the estimated wind speed (νe) and the constant rotor speed (ωc), wherein correcting the blade pitch angle (θ) in the reference look-up table further comprises a further calibration cycle, the further calibration cycle comprising determining if a further current power (Pa) is a further available maximum power (Pmax) of the wind turbine at a further point in time, based on a further determined estimated wind speed (νe) and the power curve of the turbine, wherein the power curve indicates the available maximum power (Pmax) at different wind speeds (ν), then derating the power production of the turbine until the wind turbine produces again a further limited power (Pl), wherein the derating of the power production of the turbine to the further limited power comprises derating the power production of the further limited power (Pl) by changing the current blade pitch angle (θa) and by keeping the rotor speed (ωa) constant, measuring the further current blade pitch angle (θa) and calculating a further calculated blade pitch angle (θt) at the further limited power (Pl), wherein the further calculated blade pitch angle (θt) is calculated by the further limited power (Pl), the further estimated wind speed (νe) and the further constant rotor speed (ωc), modelling a transfer function on the basis of a value of the blade pitch angle in the reference look-up table, wherein the transfer function is generated by polynomial regression on the basis of a) an offset (a) between the measured blade pitch angle (θm) and the calculated pitch angle (θt), and b) a further offset (a) between the further measured blade pitch angle (θm) and the further calculated pitch angle (θt), wherein the transfer function is adapted for determining a corrected blade pitch angle (θc) in the reference look-up table. a) an offset (a) between the measured blade pitch angle (θm) and the calculated pitch angle (θt), and b) a further offset (a) between the further measured blade pitch angle (θm) and the further calculated pitch angle (θt),