Wattmeter

A wattmeter is an electrical instrument that measures the real power, in watts, consumed by an electrical circuit. Real power is the average value of instantaneous power and equals VI cos φ, where V is voltage, I is current, and φ is the phase angle between them. In alternating-current circuits with sinusoidal waveforms, wattmeters measure active power; digital models can also report reactive power (Q), apparent power (S), and power factor (cos φ).

Most traditional wattmeters use two electromagnet coils: a current coil connected in series with the load and a voltage coil connected across the supply. The magnetic torques generated by the two coils produce a deflection that is proportional to the average product of voltage and current, after calibration. Modern wattmeters are digital and sample voltage and current to compute the instantaneous product and then average it to obtain active power; these devices can provide true-RMS measurements and are capable of handling harmonic content.

Types include electrodynamometer wattmeters (the classic moving-coil design), moving-iron wattmeters (robust and inexpensive but with limited accuracy for non-sinusoidal waveforms), and digital or solid-state wattmeters that use microprocessors. In three-phase systems, two-wattmeter methods can measure total active power, while modern meters can compute P, Q, S, and cos φ directly from per-phase samples and are common in power monitoring and energy metering.

Applications include laboratory power measurement, testing of electrical equipment, building and facility energy monitoring, and integration into smart meters and power quality analyzers. Calibration and maintenance align with standards to ensure accuracy, and proper burden and safety considerations apply.