What instrument do you use to measure true power?

The wattmeter is the instrument specifically designed to measure true power in an electrical circuit. True power, measured in watts, refers to the actual power consumed by a device, which takes into account the resistive and reactive components of the load. The wattmeter achieves this by simultaneously measuring voltage and current, then calculating the power factor to provide an accurate reading of real power usage.

In contrast, while an ammeter measures current (in amperes) and a voltmeter measures voltage (in volts), neither can directly determine true power without additional information about the phase relationship between voltage and current. A multimeter, though versatile and able to measure various electrical parameters including voltage, current, and sometimes power, does not specifically measure true power unless it has the capability to account for the power factor and calculate wattage. Thus, the wattmeter remains the only instrument capable of accurately measuring true power in a circuit.