Reactive Power (kVAR) Calculator
Calculate reactive power using the method you already have data for. This tool also estimates apparent power, phase angle, and optional power factor correction.
Q = P × tan(cos-1(PF))
Q = √(S² − P²)
Ssingle-phase = V × I / 1000, Sthree-phase = √3 × V × I / 1000
What Is Reactive Power?
Reactive power is the portion of electrical power that moves back and forth between source and load, rather than being converted into useful work. It is measured in kilovolt-ampere reactive (kVAR). Motors, transformers, welders, and fluorescent lighting all require reactive power because they use magnetic or electric fields internally.
In practical terms, active (real) power in kilowatts (kW) does useful work such as turning a motor shaft, while reactive power supports the electromagnetic conditions needed for that work. Apparent power (kVA) is the vector combination of both.
Why This Matters in Real Facilities
- Lower utility penalties: Many commercial tariffs apply extra charges for poor power factor.
- Reduced line losses: Excess reactive current increases I²R losses in cables and transformers.
- More available capacity: Better PF can free up existing transformer and feeder capacity.
- Improved voltage regulation: Reactive compensation can help stabilize system voltage under load.
How to Use This Reactive Power Calculator
Method 1: Real Power + Power Factor
Use this when you know the load kW and PF from a meter or equipment spec. The calculator computes the phase angle and resulting reactive power directly.
Method 2: Apparent Power + Real Power
Use this when your instrument reports kVA and kW. Reactive power is found through the power triangle: Q = √(S² − P²).
Method 3: Voltage + Current + PF
Use this when you have measured electrical quantities and PF. For three-phase systems, the apparent power formula includes √3.
Interpreting the Results
After calculation, you’ll see:
- Reactive Power (Q): kVAR currently demanded by the load.
- Apparent Power (S): Total VA burden seen by the source.
- Real Power (P): Useful power converted to work or heat.
- Power Factor (PF): Ratio P/S, between 0 and 1.
- Phase Angle (φ): Degree separation between voltage and current.
If you enter a target PF, the calculator estimates capacitor kVAR needed to shift from existing PF to target PF.
Example
Suppose a plant feeder draws 120 kW at 0.80 PF.
- φ = cos-1(0.80) ≈ 36.87°
- Q = 120 × tan(36.87°) ≈ 90 kVAR
- S = 120 / 0.80 = 150 kVA
If the target PF is 0.95, required capacitor compensation is approximately:
Qc = P × (tan φold − tan φnew) ≈ 50.6 kVAR
Power Factor Correction Tips
Capacitor Bank Placement
- Centralized banks are easier to maintain.
- Distributed correction near motor control centers often reduces feeder current more effectively.
Avoid Overcorrection
Overcompensating can push PF leading, potentially causing overvoltage or resonance issues. Always validate with utility requirements and harmonic studies where needed.
Consider Harmonics
If variable frequency drives or nonlinear loads are significant, detuned or filtered capacitor banks may be safer than plain capacitor steps.
Frequently Asked Questions
Is higher kVAR always bad?
Not necessarily. Reactive power is essential for inductive equipment. The goal is balance: enough reactive support for operation without excessive grid burden.
What is a good target power factor?
Most facilities target 0.95 or better, but the right value depends on tariff structure, system behavior, and economics.
Can this replace a full electrical study?
No. This is a practical engineering estimate tool. For large projects, perform detailed load flow, short-circuit, and harmonic analysis.