kVA to kW \u2014 Power Factor, Sizing and the Math Electricians Live By
Blog › Engineering · 10 min read · Published 2026-06-01
Convert between kVA, kW and kVAR with confidence. Understand power factor, generator sizing, motor loads and how to avoid undersizing your switchgear.
The Apparent / Real / Reactive Triangle
Every AC electrical system has three power values that look similar but mean different things. Apparent power (S, kVA) is what the conductors and transformers carry. Real power (P, kW) is what does useful work — heating, lighting, mechanical output. Reactive power (Q, kVAR) is the power sloshing back and forth between inductive loads and the source. They relate via the power triangle: S² = P² + Q². The kVA to kW Calculator handles the conversion with a single power-factor input.
Power Factor — The Cosine That Bills You
Power Factor (PF) = P / S = cos(φ), where φ is the phase angle between voltage and current. A purely resistive load (heater, incandescent bulb) has PF = 1. Induction motors typically run 0.7–0.9. Modern LED drivers and switch-mode supplies can be 0.6 if unregulated or 0.95+ if PFC-corrected. Utilities penalise commercial customers whose PF falls below ~0.90 because the utility's wires still need to carry the reactive kVAR even though they bill kWh.
Converting kVA to kW
The conversion is one line: kW = kVA × PF. A 100 kVA generator running a 0.8-PF load delivers 80 kW of real power. The other 60 kVAR is reactive — the generator's alternator still has to swing it, but the engine doesn't burn fuel for it. The Three-Phase Power Calculator handles three-phase variants: P (kW) = √3 × V × I × PF / 1000 for line-to-line voltage.
Converting kW to kVA — Sizing for the Worst Case
Going the other way is where errors happen. kVA = kW / PF. A 50 kW motor at PF = 0.7 needs 71.4 kVA of supply capacity — 43% more than its kW rating. Undersize the switchgear or generator to the kW figure and the breakers will trip every time the motor hits steady state. Generators are always sold in kVA for this reason.
Generator Sizing — The Three Rules
First, size to the worst-case kVA, not kW. Second, allow for motor inrush — induction motors draw 5–7× rated current for 0.5–2 seconds at start. A 30 kVA standing load can briefly demand 180 kVA inrush. Third, allow 25% headroom for capacity growth and load imbalance. Use our Generator Size Calculator with the largest single motor entered as 'starting load' to get a defensible size.
Power Factor Correction — Capacitors Pay For Themselves
Adding capacitors in parallel with inductive loads cancels reactive power. The capacitor stores energy on each half-cycle that the motor needs on the next half-cycle, so the utility no longer has to swing it. A typical PF correction project lifts site PF from 0.78 to 0.95 and pays back in 12–24 months from avoided utility penalties. The required kVAR is: Q_cap = P × (tan(φ₁) − tan(φ₂)), where φ₁ is the existing angle and φ₂ is the target.
Three-Phase vs Single-Phase Formulas
Single-phase: P = V × I × PF. Three-phase: P = √3 × V_LL × I × PF, where V_LL is line-to-line voltage. Common gotcha: V_LL = √3 × V_phase. Plug line-to-neutral voltage into the line-to-line formula and you'll undersize by 73%. The Three-Phase Power Calculator takes voltage type explicitly to remove the ambiguity.
Motors and HP
Mechanical horsepower output relates to electrical input through motor efficiency: HP_out = (P_in × η) / 0.746. A 10 HP motor at η = 0.88 draws 8.47 kW. That's the kW figure that goes into your kVA sizing — not the nameplate HP. The kW to HP Calculator handles the bidirectional conversion for both metric and mechanical HP.
Common Load Power Factors
Resistive heating: 1.0. Incandescent / halogen: 1.0. LED with PFC driver: 0.9–0.99. LED without PFC: 0.5–0.7. Induction motor at full load: 0.85. Induction motor at 25% load: 0.5. VFD-driven motors: 0.95 (input side, after the rectifier). Welding equipment: 0.5–0.7 with massive inrush. Air conditioners (residential): 0.85–0.9. Use these for first-pass sizing if datasheets aren't to hand.
Reading a Generator Nameplate
A nameplate like '125 kVA / 100 kW @ 0.8 PF, 400/230 V 3-phase, 180 A' tells you: maximum apparent power 125 kVA, real power capability 100 kW at the rated PF, voltage line-to-line 400 V and line-to-neutral 230 V, full-load current 180 A per phase. Run any load at a PF below 0.8 and you'll hit the 125 kVA alternator limit before reaching 100 kW. Run at PF = 1.0 and the engine becomes the limit at 100 kW (the alternator could carry more).
Bottom Line
kVA is what the wires see, kW is what the work gets. Power factor is the bridge. Size every supply, breaker and generator on kVA, bill every kilowatt-hour on kW, and you'll avoid 95% of the sizing mistakes that knock out commercial buildings. Run your scenarios through the kVA to kW Calculator and Generator Size Calculator before ordering switchgear.