Apparent Power

Formula

Description

Apparent power is the vector sum of real power (which does useful work) and reactive power (which shuttles energy back and forth between source and load). It represents the total power that must be supplied by the source and transmitted through the wiring, even though only the real power component performs useful work. Transformers, cables, and generators must be rated for apparent power, not just real power. The power triangle relates S, P, and Q through the Pythagorean theorem, with the power factor angle between P and S.

Variables

• S — Apparent power in volt-amperes (VA)
• P — Real (active) power in watts (W)
• Q — Reactive power in volt-amperes reactive (VAR)

Practical Notes

Units are deliberately different: watts for real power, VAR for reactive, VA for apparent. A 1000 VA transformer with a 0.8 power factor load delivers only 800W of real power. The remaining 600 VAR circulates as reactive power. UPS systems and generators are rated in VA to account for worst-case power factor loads.