The determination of the efficiency of an electric motor or an inverter in an electric vehicle requires power measurements on the ac side of the inverter. Voltage and current signals arise there from the switching operation of the inverter and are therefore hardly bandlimited. The finite bandwidth of measurement devices, like sensors or ADCs, yields to a bandwidth limitation of the measured signals. This limitation modifies the measured signals and has an effect on the calculated power. Measurement uncertainty considerations require the evaluation of the relation between this bandwidth limitation and the resulting error in the power measurement. This relation allows then for evaluating the significance of the spectral components for the power calculation and the decision which components are relevant and must not be filtered out by a bandwidth limitation. This paper presents an evaluation method for determining the bandwidth limitation error and identifies critical quantities in a numerical study.