Distortion of the supply voltage undoubtedly reduces the overall energy efficiency of induction motors. The goal of this paper is to evaluate additional harmonic losses in relation to the machine's initial efficiency rating. However, due to the relatively small amount of additional loss, the main drawback when evaluating the effect of supply voltage distortion on the machines' overall efficiency is the measurement accuracy of the current methods. In order to validate the effect of motor efficiency rating in relation to supply voltage distortion, the use of the parameter "harmonic active power" is suggested. This new strategy is subsequently validated by measurements and compared to current methods such as the NEMA MG1, or the procedure according to the IEC60034-2-3. Although this strategy is only valid under certain constraints, this new method enables a fast and highly accurate measurement of the additional loss. Consequently, this parameter is used to validate the effect of efficiency rating of induction motors in relation to the additional loss related supply voltage distortion.