AC electrical systems are often described with three related power values: real power, reactive power, and apparent power. Understanding the difference helps technicians interpret equipment ratings, power factor, electrical loading, and measurements.

Real power

Real power (P) is the portion of electrical power that performs useful work. It is measured in watts or kilowatts (kW). Heating elements, lighting, computing equipment, and motors ultimately consume real power to produce useful output.

Reactive power

Reactive power (Q) is associated with energy exchanged between the source and reactive components such as inductors and capacitors. It is measured in volt-amperes reactive (VAR). Motors, transformers, and other inductive loads can require reactive power to establish magnetic fields.

Apparent power

Apparent power (S) represents the combined effect of real and reactive power and is measured in volt-amperes (VA) or kilovolt-amperes (kVA). Electrical distribution equipment is commonly rated in VA or kVA because conductors, transformers, UPS systems, and generators must carry the total electrical load rather than only the useful watts.

Power factor

For a simple sinusoidal case, power factor can be expressed as PF = P ÷ S. If a load uses 8 kW of real power while drawing 10 kVA of apparent power, its power factor is 0.8.

Think of apparent power as the total electrical capacity being carried and real power as the portion doing useful work. Power factor describes how effectively that apparent-power capacity is being converted into real power.

Technician exercise

A device reports 12 kW of real power and 15 kVA of apparent power. Divide 12 by 15. The resulting power factor is 0.8. Next, inspect the nameplates of a UPS, transformer, generator, or other AC equipment and identify whether its rating is expressed in W, kW, VA, or kVA.

Field takeaway

Do not treat kW and kVA as interchangeable. Real power describes useful energy conversion, while apparent power describes the total AC loading that equipment may need to carry. Power factor connects the two.