Power Factor

Power factor is a comparison of the power used by the load, called "real power," divided by the power supplied to the load, which is called "apparent power." The difference between the real power and the apparent power is called "reactive power." Reactive power performs no useful work but it must be supplied to the customer in order for motors and other inductive loads to operate.

Power factor is essentially the ratio of the useful work performed by an electrical circuit compared to the maximum useful work that could have been performed at the supplied voltage and amperage. Low power factors can be a problem for some customers because their electrical distribution system may not have adequate current-carrying capacity. In fact, customers frequently run out of distribution capacity because they didn't consider power factor into their original circuit design. In addition, some utilities have significant economic penalties for low power factors.

Resistive loads, like light bulbs and heaters, have power factors of 1.0 which is called the "ideal" power factor. Inductive loads like motors have power factors of less than 1.0, usually between 0.5 and 0.95, depending on their size and how they are operated.

Power factor is expressed as the ratio of real power to apparent power, as shown by the equation: power factor equals real power divided by apparent power

Where:
Real power is measured by a wattmeter, and apparent power is measured by a voltmeter and ammeter. Power factor is important because the power supplier must supply both real and reactive power to meet the customers needs. Customers are sometimes only billed based on the real power they use. Standard utility meters measure real power and cannot measure reactive power without special modifications. A low power factor is generally considered to be anything less than an 80 to 90 percent power factor rating.