The minimum number of wattmeter(s) required to measure 3 $$-$$ phase, 3 $$-$$ wire balanced or unbalanced power is

Electro dynamic type watt meters have large errors while measuring power in a.c circuits at low power factor conditions, since the voltage across and the current though the

A dynamometer type wattmeter responds to the

A water boiler at home is switched on to the $$A.C$$ mains supplying power at $$230$$ $$V/50$$ $$Hz.$$ The frequency of instantaneous power consumed by the boiler is