Abstract: The method involves use of an instrument with at least one operational amplifier to sense the analog value first of a voltage, and then of the relative current, introducing a known delay and applying a reference voltage at the amplifier input to render all the signals positive. The analog values are then converted to digital voltage and current values and are corrected by subtracting the offset value of the operational amplifier, which is given by averaging the digital values. The instrument includes an analog-digital converter as one component of a data processing unit, which further incorporates a switch, a sample-and-hold circuit, a memory with an internal shift register, and a programmable control unit which is capable of shifting the compensated current into quadrature in such a manner as to allow a direct calculation of reactive power by way of the shift register.