Atmospheric turbulenceTurbulence occurs on a spectrumSpectrum of spatial and temporal scales and has important impacts on wind energyWind energy. TurbulenceTurbulence causes structural fatigue loads on wind turbinesWind turbine but it also has a significant impact on the power output. In this study, we investigate the effect of turbulenceTurbulence on wind turbineWind turbine power output. We show how the rotor dampens the effect of the smaller scales of turbulenceTurbulence leading to a low-pass filter effect on temporal fluctuations of power output. This spatial filter effect on turbulenceTurbulence is well known within wind loadsWind loads on structures where it is characterized by the background response factor that accounts for the non-coherenceCoherence of a turbulent windTurbulent wind field across the structure. However, in the IEC 61400-12-1 standard for wind turbineWind turbine power curvePower curve verification, this spatial rotor-filter effect is omitted, and turbulent windTurbulent wind fields are implicitly assumed to be fully coherent. Errors on the resulting rotor-filtered wind speed variance increase with the rotor diameter, exceeding 50% for modern rotors of 100 m or more. We present a new method to efficiently predict the rotor-filter effect as a function of rotor size. The proposed method is validated against aero-elastic simulations and field measurementsField measurements, both demonstrating a strong agreement.