Power Factor (PF)

This term is used to express the relationship between "useless current" and "useful power". It can be very confusing to explain and understand. Certain types of electrical devices have a power factor of 100%, such as an electric stove, a light bulb, toaster, etc., which means when the appliance is on, all available power is being used to heat or illuminate and none is being wasted. Some other devices, especially induction motors as commonly used today, are not being used at capacity and result in a demand on the system greater than actually being used or put to good use. The actual work being done by the motor results in a certain kilowatt (kW) demand that is measured by the ordinary meters for measuring such demand. This motor, however, when "partially" loaded, makes an additional demand on the electric system which is not measured by the ordinary meter, but such additional demand requires capacity in the electric system in just the same way as the useful demand requires capacity. When there is no useless current in evidence, the power factor is said to be in "Unity".

Power Factor is normally used in calculating kilowatts by the expression kW = kVA x PF. To compute power factor, the expression would be:

PF = kW/kVA or (W/(E x I)).

If an electric motor requires 100 kilowatts of useful power and is operating at 50% power factor, the above formula would yield as follows: 100 kW = kVA x .50 PF. To solve for kVA, kVA =100 / .5 = 200. In other words, this motor requires 200 kilovolt-amperes (kVA) of capacity in the electric system although it only uses 100 kW of useful power. The electric system is still having to provide 200 units of capacity in transformers, lines, etc. to serve that motor. If power factor for that motor could be increased to "unity", the motor would do no more useful work, it would take no more energy to perform this work, but would make a demand of 100 kw on the electric system, and only 100 kw in capacity in the electric system would be required to serve the motor. If that same 100 kw motor is now working at 70% power factor, the kVA required would be 143, or 100 / .7. An improvement over the 200 previously required. The higher the power factor of a load, the better it is to serve.

Generally, if a customer pays an electricity bill with units of energy measured askVA, then the customer will benefit from savings by increasing power factor. If the customer pays an electricity bill with units of energy measured as kW, then the utility company will benefit from savings by increasing power factor. Frequently, however, utility companies impose a power factor penalty charge on customers with poor PF, giving the customer an economic incentive to increase power factor, even if the customer is billed based on kW demand.