Wattmeter

[citation needed] In 1974 Maghar S. Chana, Ramond L. Kraley, Eric A. Hauptmann Barry, and M. Pressman patented an early electronic wattmeter.

Also, on analog wattmeters, the potential coil carries a needle that moves over a scale to indicate the measurement.

The potential coil has, as a general rule, a high-value resistor connected in series with it to reduce the current that flows through it.

For AC power, current and voltage may not be in phase, owing to the delaying effects of circuit inductance or capacitance.

On an AC circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring active power, P=VI cos φ.

For each sample, the voltage is multiplied by the current at the same instant; the average over at least one cycle is the real power.

The readings may be displayed on the device, retained to provide a log and calculate averages, or transmitted to other equipment for further use.

Wattmeters vary considerably in correctly calculating energy consumption, especially when real power is much lower than VA (highly reactive loads, e.g. electric motors).

[4] If a laptop computer in sleep mode consumes 5 W, the meter may read anything from 0 to 15.25 W, without taking into account errors due to non-sinusoidal waveform.

In practice accuracy can be improved by connecting a fixed load such as an incandescent light bulb, adding the device in standby, and using the difference in power consumption.