This paper describes the results of a study that looked at how harmonic voltage and current distortions affect the power metering equipment of an aluminium plant. Aluminium industry power supply circuits and power consumers have been analyzed. The main requirements are specified with regard to power monitoring systems designed for major consumers. It is shown that when calibrating commercial-type digital power meters no testing is required for the conditions of voltage or current distortions. At the same time, it is in these conditions that such devices are utilized at feeder points with non-linear loads. As a result of a series of laboratory studies, it was found that the number of the considered harmonics, their amplitude and the phase displacement angle at 50 Hz produce most effect on the active and reactive power monitoring error. The authors also analyzed mathematical models of the power and electric power meters in relation to the active and reactive power calculated in the presence of higher harmonics. It is shown that different mathematical equations can be used to calculate the reactive power using the current metering devices. The paper presents some measurement data on the electric power quality and the power consumption parameters from the compressor stations and the process water plants of an aluminium plant. The data show that the total voltage harmonics go beyond the allowable limit, and the distortions in that point are caused by variable frequency converters of the electrolysis facility. The measured parameters were used as the basis for estimating the error of the electric power meters for various mathematical models implemented in the meters. It was found that the discrepancy between the first harmonic method and the geometry method in the reactive power calculation can reach 6%.This research was funded by the Russian Science Foundation, Project No. 18-79-00127.