Power Factor

Power factor is not a measure of efficiency. It is a ratio of Real Power, in total kilowatts, to total Apparent Power, in kilovolt amps. If a load consumes Reactive Power, the power factor is said to be "lagging." Most electric motors have a lagging power factor. The operation of electrical systems with low power factor results in voltage increases and power losses, reducing the overall power carrying capacity of the power supply system. As an incentive for customers to operate at high power factors, utilities assess power factor penalties to customers whose overall power factor falls below certain levels.

An understanding of the difference between the three aspects of power, kilowatts, kilovolt-amps, and kilovolt-amps reactive, is essential to an understanding of power factor. Useful mechanical work derives from "real power," the energy consumed by the load. Real power is expressed in kilowatts. Measured by a standard watt hour meter or kilowatt demand meter, Real Power is what utilities bill to customers as "Demand Charge billed dollars per kilowatt."

Reactive power does not provide useful mechanical work. However, most AC motors do require reactive power for developing magnetic fields. Reactive power is expressed as Kilovolt-amps Reactive, or kVAR. The sum of Real Power and Reactive Power is Apparent Power, expressed as kilovolt-amps or kVA. Apparent Power is calculated by multiplying voltage times amperage.

Power factor is expressed as a decimal number less than or equal to 1.0. A power factor of 1.0 indicates that Real and Apparent Power are equal, so no Reactive Power consumption is taking place. True resistance loads, such as incandescent light bulbs and water heater elements, often have power factors of 1.0. Power factors vary among motors, ranging between 0.5 and 0.95. This figure depends mostly on a given motor's size, construction, and load.

And Furthermore . . .