When it comes to power, not all electrical appliances are created equal. To find out how much power an appliance consumes, energy auditors occasionally multiply line voltage by the current reading obtained from a clamp-on ammeter. However, depending on the appliance, this simple calculation will not always reflect true power usage. Utilities bill residential customers only for true energy use (that is, for the time integral of true power). In performing residential energy audits, it is therefore important to know how to measure true power.

The problem is that some loads draw more current than others to deliver a given amount of usable power. To determine the actual power consumed by an appliance, it is necessary to take account of its power factor.

The power factor is the value of the actual (or true) power used by an appliance divided by the expected (or apparent) power--the latter being the product of the line voltage (V) and current (I).

Power factors range between zero and one; a value of one indicates that all the electricity drawn by a load is being consumed. Electric-resistance heaters and incandescent bulbs are examples of appliances that have a power factor of one. Taking a 60W light bulb as an example, the wattage represents the actual power consumed by the bulb. Figure 1 shows the light bulb connected to a standard 120V outlet with an ammeter measuring 0.5 amperes flowing through the circuit. In this case, the 60 watts of power could be calculated by multiplying 120 volts by the 0.5 ammeter reading.

Electric motors, fluorescent lamps, and consumer electronics (such as televisions and computers) are examples of appliances that have power factors of less than one. This is because they include some type of storage element--such as a capacitance or inductance--or create distortions in the voltage/ current waveforms.

In addition to providing a quantity of horsepower, the motor used to run a refrigerator compressor will temporarily store electricity in its inductor--that is, in the turns of wire wrapped around the metallic core. The motor's power factor is reduced because the load draws current that is not temporally in phase with the voltage waveform.

Figure 2 shows the supply voltage for such a motor at 120 volts and the measured current at 5 amperes. Notice that the product of these values (600 volt-amps) does not equal the power measured by the wattmeter (400 watts). This is because the motor inductor is storing current and then returning it back to the line. The amount of returned current depends on how much work is being done--that is, how much mechanical energy (horsepower) is being performed by the motor. In this case, the power factor is 0.67 or 67% (400/600). This means that the motor requires 50% more current for a given amount of power than a load with the same true power and a power factor of one.

Incongruent waveforms associated with inductor motors can be corrected with synchronizing devices, such as capacitors. However, low power factor can also be caused by distortions in the shape of the waveforms, known as harmonic distortion. Such problems are commonly associated with consumer electronics and electronic ballasts and are not as easily corrected (see Understanding Power Quality, HE Nov/Dec '93 p. 33).

If you want to measure true power, and you are unsure of an appliance's power factor, it is better to use a wattmeter than to try to calculate power usage from an ammeter reading.