The measurement of electrical power requires that voltage and current both be measured simultaneously. Remember that in a dc circuit, the power (P ) in watts is the product of the voltage (E ) in volts and the current (I ) in amperes. That is, P = EI. In fact, watts are sometimes called volt-amperes in dc circuits.
Do you think you can connect a voltmeter in parallel with a circuit, thereby getting a reading of the voltage across it, and also hook up an ammeter in series to get a reading of the current through the circuit, and then multiply volts times amperes to get watts consumed by the circuit? Well, you can. For most dc circuits, this is an excellent way to measure power, as shown below.
In a dc circuit, power can be measured with a voltmeter and an ammeter, connected as shown here.
Sometimes, it’s simpler yet. In many cases, the voltage from the power supply is constant and predictable. Utility power is a good example. The effective voltage is always very close to 117 V. Although it’s ac, and not dc, power in most utility circuits can be measured in the same way as power is measured in dc circuits: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done. Then, rather than 1 A, the meter will show a reading of 117 W, because P = EI = 117 × 1 = 117 W. If the meter reading is 300 W, the current is I = P/E = 300/117 = 2.56 A. An electric iron might consume 1000 W, or a current of 1000/117 = 8.55 A. A large heating unit might gobble up 2000 W, requiring a current of 2000/117 = 17.1 A. You should not be surprised if this blows a fuse or trips a circuit breaker, because these devices are often rated for 15 A.
Specialized wattmeters are necessary for the measurement of radio-frequency (RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized applications. But almost all of these meters, whatever the associated circuitry, use simple ammeters, milliammeters, or microammeters as their indicating devices.