The concept of active power measurement based on real-time digital processing of voltage and current signals is best’ introduced with a digital approximation definition . In this paper, the authors propose an integral calculation method based on Gregory’s expression, correction of integration period estimation and integration result correction taking values related to the ends of the integration period into account. In the described method, the integral is calculated between two consecutive maximums of the voltage and current product.. Digital dependencies that are easy to implement are used for the evaluation of the active power value. To measure the voltage and current signals, the authors used a system with a digital signal processor and an A/D converter with a multiplexer followed by dedicated digital filters. The active power measurement method was checked by building an instrument simulation model.