Power curves for offshore wind turbines within the German offshore wind farm alpha ventus were derived based on the IEC standard. Binning in groups of shear and turbulence intensity as measures of atmospheric stability were performed. The derived power curves show a strong dependency on these two parameters. Differences of up to 15% in power output between unstable and stable stratification in the non-wake case occur. For wind turbines within the wake of others the effects are even more pronounced. Here, the differences in power production between the stability classes approach 20%. This dependency of the power curves on stability can cause significant miscalculations of instantaneous power production, long-term energy yield and loads. Parameters other than the hub height wind speed are often not taken into account in state-of-the-art wind power forecasts. This can lead to substantial over- or underestimation of the resulting power.