Wattmeter

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The wattmeter is an instrument for measuring the electric active power (or the average of the rate of flow of electrical energy) in watts of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.

A wattmeter reads the average value of the product v(t)i(t) = p(t), where v(t) is the voltage with positive reference polarity at the ± terminal with respect to the other terminal of the potential coil, and i(t) is the current with reference direction flowing into the ± terminal of the current coil. The wattmeter reads P = (1/T) ∫₀^T v(t)i(t) dt, which in sinusoidal steady-state reduces to V_rms I_rms cos(φ), where T is the period of p(t) and φ is the angle by which the current lags the voltage.^[1]