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Average electric power can be measured by a system that samples voltages and currents at predetermined intervals. The sampled signals are digitized and the result is computed by numerical integration. The response of the system agrees with that of a standard electrodynamic wattmeter within 0.02 percent from dc to 1 kHz, with the possible exception of zero power factor measurements. Measurements up to 5 kHz can be made with somewhat greater uncertainties.