Reactive Power

Formula

Description

Reactive power represents the component of apparent power that oscillates between source and load without performing useful work. It is caused by energy storage elements (inductors and capacitors) that alternately absorb and release energy each half cycle. Inductive loads (motors, transformers) consume positive reactive power, while capacitive loads generate it. The phase angle φ between voltage and current determines the split between real and reactive power. At φ = 0 (purely resistive), Q = 0; at φ = 90° (purely reactive), all power is reactive.

Variables

• Q — Reactive power in volt-amperes reactive (VAR)
• S — Apparent power (VA)
• φ — Phase angle between voltage and current (degrees)

Practical Notes

Power factor correction adds capacitors to cancel inductive reactive power, bringing the net reactive power closer to zero and the power factor closer to unity. Industrial capacitor banks are sized in kVAR. Over-correction (leading power factor) can cause voltage rise and is usually avoided. Some utilities charge separately for reactive power consumption above a threshold.