What instrument is used for measuring electric power?

• A. Wattmeter
• B. Ammeter
• C. Voltmeter
• D. Ohmmeter

Answer: (A)

The essential idea is that measuring electric power requires knowing both voltage and current, since power is the rate at which energy is transferred and in AC circuits it depends on V × I × cos(phi). A wattmeter is designed to sense both the voltage across a load and the current through it and convert that information into a reading in watts, effectively capturing the real power being delivered. Its construction places a current path in series with the load and a voltage path across the load, and the magnetic interaction between these paths yields a deflection proportional to the product of voltage and current (accounting for power factor). In contrast, a voltmeter only measures voltage, an ammeter only measures current, and an ohmmeter measures resistance, so they cannot directly indicate power. Therefore, the instrument that provides a power reading is the wattmeter.