The meter that measures true power in a circuit is the wattmeter.

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

Answer: (A)

Real power is the portion of electrical power that actually does work in the circuit, and in AC circuits it depends on how much the voltage and current are in phase. The wattmeter is designed to measure true power by incorporating both the voltage and the current (and their phase relationship) into its reading, giving you power in watts. In other words, it accounts for the cosφ factor in P = VI cosφ, so it shows the actual work the circuit can do.

A voltmeter reads only voltage, an ammeter reads only current, and an ohmmeter measures resistance. Those meters don’t directly tell you how much real power is being delivered, especially when there’s a phase difference between voltage and current.