Reactive Power Calculator
Reactive power (kVAR) is the component of apparent power that shuttles between source and load without doing work. Capacitors supply it; inductors absorb it.
Common conversions
| Input | Result |
|---|---|
| 5 kW @ PF 0.95 | 1.64 kVAR |
| 10 kW @ PF 0.90 | 4.84 kVAR |
| 10 kW @ PF 0.85 | 6.20 kVAR |
| 20 kW @ PF 0.80 | 15.00 kVAR |
| 25 kW @ PF 0.75 | 22.05 kVAR |
| 50 kW @ PF 0.70 | 51.01 kVAR |
| 100 kW @ PF 0.60 | 133.33 kVAR |
| 100 kW @ PF 1.00 | 0 kVAR |
The math behind it
- arccos(0.78) = 38.74°
- tan(38.74°) = 0.804
- kVAR = 18 × 0.804
Everything you need to know
Reactive power (kVAR) is the portion of apparent power that shuttles between source and load every half-cycle without ever converting into work, heat, or light. It exists because inductors and capacitors store energy in magnetic and electric fields and hand that energy back to the circuit a moment later, rather than consuming it the way a resistor does.
The power triangle
Real power (kW), reactive power (kVAR), and apparent power (kVA) form a right triangle: kW along the base, kVAR along the vertical side, and kVA as the hypotenuse. The angle φ between kW and kVA is the same angle by which current lags or leads voltage, and cos φ equals the power factor. Because it's a right triangle, kVA² = kW² + kVAR² always holds, so knowing any two of the three values gives you the third.
What causes reactive power
Inductive loads, motors, transformers, magnetic ballasts, and induction welders, absorb lagging reactive power to build the magnetic fields they need to operate. Lightly loaded or idling motors and transformers are often the worst offenders, since their magnetizing current stays nearly constant even as real power output drops toward zero. Capacitive sources work the opposite way: correction capacitor banks, long or lightly loaded underground cable runs, and some electronic power supplies generate leading reactive power. Pairing inductive and capacitive sources cancels their reactive flow against each other, which is exactly how power factor correction works.
Why it doesn't show up as billed energy, but still costs money
Residential and most commercial meters bill kWh, real energy, because reactive power dissipates no heat and performs no work, so it isn't "consumed" in the way a bill measures. But the current needed to carry that reactive power still flows through every wire, breaker, transformer, and generator winding in the path, adding I²R losses and forcing all of that equipment to be rated larger than the real power alone would require. Utilities recover this hidden cost indirectly, through a power factor penalty clause or by switching a low-PF account to demand billing based on kVA instead of kW, rather than metering kVARh directly. Reactive power is effectively free only until a facility's PF drops low enough to trigger one of those charges.
Common applications
Even when kVAR itself isn't billed directly, the low PF it causes routinely triggers a power factor penalty clause or shifts billing to kVA demand instead of kW demand, both of which raise a commercial account's monthly cost.
Calculating existing kVAR at the current PF, then subtracting the kVAR needed at a target PF, gives the exact capacitor bank size required for a correction project.
Reactive power adds directly to the current a generator or transformer must supply. A 500 kVA transformer serving a 350 kW load at PF 0.7 already carries roughly 357 kVAR of reactive current, close to its thermal limit even though the real power looks moderate.
Measuring kVAR alongside kW during commissioning flags oversized or lightly loaded equipment, since magnetizing kVAR stays high even when real power output is low.
Common mistakes
It isn't billed as energy on most accounts, but the extra current it creates causes real I²R losses and often triggers a separate power factor penalty or kVA demand charge elsewhere on the bill.
kVAR is an instantaneous rate; kVARh is accumulated reactive energy over time. Mixing the two produces numbers off by whatever time period was used, the same error as confusing kW with kWh.
Equipment must be rated in kVA, which includes both real and reactive power. Sizing a generator to the real kW load alone while ignoring a low PF leads to overheating or nuisance shutdowns under full load.
A well-sized capacitor bank produces reactive power on purpose, to cancel the reactive power that inductive loads absorb. The goal isn't zero kVAR system-wide, it's balancing lagging and leading kVAR so the net demand on the utility stays low.
Frequently asked questions
Is reactive power wasted energy?+
No. It isn't dissipated as heat or converted into work, but it also isn't destroyed. It cycles between source and load 50 or 60 times a second, and the real cost it creates is the extra current needed to carry it, not energy loss itself.
Why does power factor matter if reactive power isn't billed directly?+
Because the current that delivers reactive power still flows through wires, transformers, and generators, forcing that equipment to be sized larger than the real power alone would require. Utilities often recover this cost through a power factor penalty or a kVA-based demand charge instead of billing reactive energy directly.
Do capacitors produce or consume reactive power?+
Capacitors produce, or source, reactive power with a leading characteristic, the opposite of inductors. That's why capacitor banks are used to cancel the lagging kVAR drawn by motors and transformers.
Can reactive power exist without real power?+
Yes. A purely reactive load, like an unloaded capacitor bank or a transformer's magnetizing branch with no load attached, draws kVAR while delivering essentially zero kW. Power factor at that operating point is close to zero.
How much reactive power does an idle motor draw?+
Roughly the same magnetizing kVAR it draws at full load, even though its real power draw falls to near zero. That's why an idling motor has a much worse power factor than the same motor running under load.
Does reactive power cause I²R losses?+
Yes. The current that carries reactive power flows through the same conductors as real-power current, and both contribute to I²R heating losses in wires, transformers, and generator windings.
What's the difference between kVAR and kVARh?+
kVAR is a rate, the instantaneous reactive power at a moment in time. kVARh is reactive energy accumulated over time, the reactive-power equivalent of kWh, occasionally metered on large industrial accounts subject to reactive energy charges.
Can I calculate kVAR without knowing kVA?+
Yes. If you know real power and power factor, kVAR = kW × tan(arccos(PF)) gives the same answer without needing a separate apparent power measurement.
Does correcting power factor reduce reactive power to zero?+
No, not usually. Correction capacitors cancel most of the lagging kVAR from inductive loads, typically targeting a PF around 0.95 to 0.98 rather than exactly 1.0, since pushing PF to unity risks over-correcting into a leading condition.
Is reactive power the same in DC circuits?+
No. Reactive power only exists in AC circuits, where voltage and current constantly change and can shift out of phase. Steady-state DC circuits have no phase shift, so power factor is always 1.0 and kVAR is zero.