In AC power measurement, which instrument is used to measure true power?

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

Answer: (B)

Real power is the average of the instantaneous product of voltage and current over time, which means you need an instrument that effectively integrates p(t) = v(t)i(t) to give the actual work done per unit time. A wattmeter is built with a voltage element and a current element arranged so their interaction produces a deflection proportional to this average, accounting for any phase difference between voltage and current. In other words, it reads P = Vrms Irms cos(phi), the true power in the circuit. An oscilloscope only shows waveforms and doesn’t directly provide a power value without extra calculation. A voltmeter measures only voltage magnitude, and an ammeter measures only current magnitude; neither alone captures how much of that energy is actually delivered to the load. So the wattmeter is the instrument designed to measure true power.