Measurement of Power and Energy
A. Simpson · 1976
For the measurement of power in d.c. circuits a wattmeter is not necessary since power is then the product of ammeter and voltmeter readings. For the a.c. circuit this is only true if the power factor is unity, and for all other conditions a wattmeter is essential since such an instrument indicates true power, that is supply voltage × current × cosine of phase angle between voltage and current