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A mathematical nonlinear model is developed to simulate the performance of an induction watthour meter for nonsinusoidal voltage and current waves. The computation results show that for the special case where the voltage is purely sinusoidal, the measurement error causedby the nonlinear magnetic circuits is generally small unless the current wave contains a relatively large third harmonic component. However, when the voltage and current waves are both nonsinusoidal, the registration error can be relatively large even for a small voltage distortion. This is attributed not only to the nonlinearity but also to the drooping frequency response curve of the meter. In both cases, the sign of the registration error depends largely on the phase angles of voltage and current harmonics with respect to their fundamentals. A comparison shows that the computation results are consistent with the experimental ones which are available in the literature.