The power consumption reduction or energy saving arising from the use of a transformer type voltage regulation device is calculated from the power induced on the primary winding from the secondary winding and a parameter based on the number of turns in, or the voltages across, the primary and secondary windings. The voltage regulation or optimization device comprises a transformer with primary and secondary windings. The power induced on the primary winding from the secondary winding is calculated by measuring the voltage on the primary winding and either the primary winding current i3 directly or by measuring the supply current i1 and the secondary current i2. The induced power may be the real power or reactive power. RMS values may be used for the voltage and current. The invention allows the effect of the voltage regulator to be monitored while in use, without needing a separate measurement of power consumed when the voltage regulator is not connected.
