Which instrument measures electrical power in watts?

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

Answer: (A)

Measuring electrical power in watts is done with a wattmeter. It directly reads real power by incorporating both the circuit voltage and current and accounting for their phase relationship. In AC systems, power is not just about how much current or how high the voltage is; it also depends on how in- or out-of-phase they are (the power factor). A wattmeter has a voltage element connected across the supply and a current element in series with the load, and its indication corresponds to P = V × I × cos(phi). A voltmeter only shows voltage, an ammeter only shows current, and a multimeter can measure those quantities but normally requires you to calculate power yourself. Hence, the instrument that measures power in watts is the wattmeter.