Reactive Power Calculator

Reactive power relates real and apparent power through the power triangle: S² = P² + Q². Here, S is apparent power (kVA), P is real power (kW), and Q is reactive power (kVAR).

• From kVA and kW: Q = √(S² − P²)
• From kW and PF: S = P / PF, then Q = P·tan(acos(PF))
• From V, I, PF: S = V·I (single-phase) or S = √3·V·I (three-phase), then P = S·PF
• Power factor correction: Qc = P·(tan(φ1) − tan(φ2)), where φ = acos(PF)

1. Select the calculation mode that matches your available measurements.
2. Choose system phase if using voltage and current inputs.
3. Enter real power, power factor, and other required values.
4. Optionally set a target power factor to estimate capacitor kVAR.
5. Press Calculate to show results above the form.
6. Use the export buttons to download your last calculated report.

Values are illustrative; always verify with site measurements.

1. Why reactive power shows up on jobsites

Reactive power matters on active jobsites because inductive loads like hoists, pumps, mixers, and welders draw magnetizing current. That current does no useful work, yet it occupies capacity in generators, transformers, switchgear, and feeder cables. Managing kVAR helps stabilize voltage and reduces nuisance trips during peak starts. It also improves motor torque during brownout conditions.

2. Reading the power triangle quickly

A practical starting point is the power triangle: S² = P² + Q². If a 55 kW crane panel runs at PF 0.85, apparent power is about 64.7 kVA and reactive power is about 34 kVAR. Those numbers quickly show whether a temporary generator has enough headroom. They also help set realistic feeder breaker sizes.

3. Current, losses, and voltage drop

Low power factor increases current for the same kW. At fixed voltage, higher current raises I²R losses in cables and produces extra heat in panelboards and connectors. Improving PF from 0.80 to 0.95 can cut current by roughly 16 percent, often extending cable run limits. Lower current also reduces voltage drop on long runs.

4. Sizing generators and transformers

Generator and transformer sizing should consider both kW and kVA. A 60 kVA unit cannot deliver 60 kW unless PF is 1.00. On construction distribution, planning with kVA prevents overload alarms when multiple motors start, and it supports realistic diversity factors for mixed loads. Add margin for future tools and seasonal pumps.

5. Estimating capacitor correction kVAR

Capacitor banks supply leading kVAR to offset lagging kVAR from motors. The correction estimate uses Qc = P·(tan φ1 − tan φ2). For 18 kW at PF 0.80 targeting 0.95, the required capacitor size is about 8 kVAR. Place banks near inductive panels to reduce feeder current.

6. Drives, harmonics, and resonance risk

Where variable speed drives are common, harmonics can interact with capacitors and shift effective PF. Detuned reactors or harmonic rated banks may be required to avoid resonance near the 5th or 7th harmonic. Coordination with a power quality survey reduces risk before installing correction. Always verify capacitor temperature and kvar drift.

7. Measuring and validating on site

Use measurements to validate assumptions. Clamp meters and power analyzers can record kW, kVA, PF, and kVAR during normal operation and motor starts. Compare logged peaks to this calculator’s results to confirm spare capacity, then update panel schedules and temporary power drawings.

8. Cost and operating impacts

Documenting reactive power also supports cost control. Many utilities apply penalties or higher demand charges when PF stays below a threshold such as 0.90. Tracking kVAR and correcting early can reduce monthly bills, free equipment capacity, and keep critical lifts running safely under load.

Q1. What is reactive power in simple terms?

Reactive power is energy that oscillates between source and inductive or capacitive equipment. It supports magnetic fields in motors and transformers but does not produce mechanical work. It is measured in kVAR and affects current and voltage stability.

Q2. Why do construction motors usually have lagging power factor?

Induction motors need magnetizing current to create a rotating field. That current lags voltage, producing lagging power factor and positive kVAR. The effect is strongest at light load and during starts, so mixed tool usage can lower overall PF.

Q3. Can I compute kVAR from voltage and current only?

Not accurately without power factor. Voltage and current give apparent power (kVA), but kVAR depends on the phase angle. If you can measure PF with a meter, you can calculate kW and kVAR reliably with this tool.

Q4. What target power factor is typical on temporary power?

Many projects aim for 0.90 to 0.95 to reduce current and avoid penalties. Going higher can risk over-correction and leading PF when loads drop. Use site measurements and adjust capacitor steps to match actual operating ranges.

Q5. Does leading power factor ever cause problems?

Yes. Excessive correction can push PF leading, which may raise voltage and create switching transients. Some generators and UPS systems also dislike leading PF. If you see leading PF at low load, reduce capacitor steps or use automatic control.

Q6. How do harmonics affect capacitor sizing?

Harmonics from drives and welders can increase capacitor current and cause overheating or resonance. Using detuned reactors and harmonic rated banks helps. A power quality survey can identify dominant harmonics and guide safer correction equipment.

Q7. Is this calculator suitable for single-phase tools?

Yes. Select single-phase and enter voltage, current, and PF, or use kW with PF. Many small tools have poorer PF under light load, so measuring PF improves accuracy. Always confirm ratings on nameplates and panels before changes.