Abstract: Power measuring apparatus including a first multiplexor having its inputs responsive to the polyphase voltage and current inputs being measured. The multiplexor reads these inputs in sequence based on address signals provided by a program controlled microprocessor. The output of the multiplexor is coupled to an RMS to DC converter, the output of which is coupled to a second multiplexor. The voltage inputs are coupled to a voltage averaging device, the output thereof being coupled to additional inputs of the second multiplexor, the output of the second multiplexor being coupled to the microprocessor via an analog to digital converter. The microprocessor reads and compares the voltage output from the averaging circuit and the corresponding voltage at the output of the converter and generates a correction factor and responds thereto.