Inverters and conductors are limited by apparent power and current—reactive kVAR is part of that budget. This guide shows how to calculate kVAR for inverter and conductor sizing from load kVA, power factor, and the real kW your motors actually draw.
Benefits
- kVAR = √(kVA² − kW²) after kW = kVA × PF.
- kVA (not kW) drives breaker and inverter nameplate checks.
- Document kVAR in BOM for motor-heavy distribution quotes.
How it works
- Sum load kVA—or use a single motor branch kVA from nameplate.
- Enter power factor (measured or motor catalog typical).
- Read kVAR and kW; verify inverter kVA ≥ load kVA, not just kW.
FAQ
How do I calculate kVAR for inverter sizing?
From kVA and PF: kW = kVA × PF, kVAR = √(kVA² − kW²). Example: 8 kVA load at 0.8 PF → kW = 6.4, kVAR ≈ 4.8. Inverter must supply 8 kVA apparent—a 6.4 kW-rated inverter may trip if its kVA limit is only 7 kVA.
Why does kVAR affect conductor sizing?
Current depends on apparent power I = kVA × 1,000 ÷ (V × √3) on three-phase (or kVA × 1,000 ÷ V single-phase). Reactive kVAR increases kVA at the same real kW, raising amps without more useful work. Size feeders on calculated kVA including kVAR.
Can I reduce kVAR for sizing?
Power-factor correction capacitors at the load or panel reduce lagging kVAR, lowering apparent kVA and current. Calculate kVAR first to see if correction is cheaper than upsizing cable or inverter kVA. After correction, re-run with improved PF.
Technical specifications
- kVAR = √(kVA² − kW²).
- kVA_sizing ≥ √(kW² + kVAR²).
- I ∝ kVA at fixed voltage (per phase rules apply).
- Related: inverter-sizing, kva-to-kw, power-factor-and-reactive-power-calculator.
kW fit is not kVA fit
Inverter datasheets list continuous kW and sometimes separate kVA or output current limits. A pump drawing 7 kW real at 0.78 PF needs about 8.97 kVA—a 8 kVA inverter is undersized even though kW looks close. Calculating kVAR exposes the apparent-power gap before purchase orders go out.
Conductors follow amps, amps follow kVA
NEC ampacity tables do not care that only part of your apparent power is real. Once kVAR is folded into kVA, convert to current at your service voltage and phase. Two loads with identical kW but different PF need different wire if PF differs—motor branch vs. heater branch on the same panel.
When kVAR drives an upsize vs. correction
If calculated kVAR is a large fraction of kVA, compare adding correction capacitors against buying the next inverter kVA tier or upsizing homerun cable. A few kVAR of correction on a fixed motor load can drop apparent kVA enough to keep existing breaker and wire—run the calculator before and after assumed PF improvement to quantify the savings.