Electronic watthour meter and power-associated quantity calculating circuit

A power-associated quantity is calculated with high accuracy by means of sampling frequency correction means for allowing the sampling frequency of the A/D converter to follow up a frequency the value of which is L times that of the power supply frequency and voltage-controller oscillator (VCO) so as to control the sampling frequency of the A/D converter with high accuracy. The reactive power used to grasp the power supply efficiency is calculated with high accuracy together with high frequency and the power factor used as an index of the efficient use of power is determined.