kW vs kVA and power factor
The short answer: kW is the power that actually does work. kVA is volts × amps, the total the generator has to push through its wiring. Power factor is the ratio between them. For heaters and lights they're the same; for motors, kVA is higher, and it's the kVA that fills up a generator.
Two ways to measure the same load
kW (kilowatts) is real power: the part that turns into heat, light or motion. It's what your electric meter bills you for.
kVA (kilovolt-amps) is apparent power: the voltage times the current flowing in the wires. A generator's windings heat up with current, so current is what limits how much it can supply.
Power factor is kW ÷ kVA, a number from 0 to 1. At 1.0 every amp does useful work. Below 1.0, some of the current flows back and forth to build the motor's magnetic field without doing work, but the generator still has to supply it.
Think of a horse towing a boat along a canal from the bank. The rope pulls at an angle, so only part of the horse's effort moves the boat forward. The forward part is kW. Everything the horse puts into the rope is kVA. The straighter the rope, the closer the power factor is to 1.
Which loads have a low power factor
- About 1.0: loads that just make heat or light, like space heaters, water heaters, ovens, toasters and incandescent bulbs.
- Below 1.0: motors and compressors, including refrigerators, pumps, furnace blowers and air conditioners, while they run.
- Much lower while starting: a motor starting from standstill draws a large current, and most of it builds the magnetic field rather than doing work. That's why motor starting is described in kVA (from locked-rotor amps), not watts. See locked-rotor amps and NEMA code letters.
What it means when you buy
Portable generators and power stations are rated in watts. Appliance labels often give volts and amps instead. Multiplying them gives volt-amps, which for a motor is a bit more than its watts. Using volts × amps as if it were watts errs on the safe side.
Home standby generators are rated in kW. Generator ratings are always stated at a power factor. The residential single-phase spec sheets we've reviewed list 1.0. That means the kW rating is also the kVA it can carry: a 22 kW unit at power factor 1.0 is good for 22 kVA. Feed it motor loads at a lower power factor and you reach its current limit before you reach 22 kW of real power.
So for a standby generator, compare your load's kVA, not just its watts, against the rating. Commercial three-phase generators are commonly rated at power factor 0.8 instead, so the same kW number means more kVA. Check the power factor on the spec sheet whenever you compare kW ratings.
The calculator adds a 25% margin to the running load for standby. For a load averaging power factor 0.8, that margin is the same size as the gap between watts and volt-amps. See the math below.
Show the math
Apparent power
kVA = volts × amps ÷ 1,000
Real power
kW = kVA × power factor
Power factor
PF = kW ÷ kVA (0 to 1)
Example: a motor nameplate says
120 V, 8 A, and it runs at PF 0.8
kVA = 120 × 8 ÷ 1,000 = 0.96 kVA
kW = 0.96 × 0.8 = 0.768 kW
A standby generator rated 22 kW
at power factor 1.0
kVA capacity = 22 ÷ 1.0 = 22 kVA
Real power it can deliver to
loads averaging PF 0.8
= 22 × 0.8 = 17.6 kW
Our standby margin vs. power factor
running kW × 1.25
= running kW ÷ 0.8
= load kVA at PF 0.8
Results are rounded up, never down.
References
- IEC 60050-131, International Electrotechnical Vocabulary, circuit theory: definitions of active power, apparent power and power factor. Free to read at Electropedia.
- IEEE Std 1459, Standard Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions.
- NEMA MG 1, Motors and Generators (synchronous generators are rated in kVA at a stated power factor).
- ISO 8528-1:2018, Reciprocating internal combustion engine driven alternating current generating sets, Part 1: Application, ratings and performance (how generator power ratings are defined).
- IEEE Std 3002.7-2018, Recommended Practice for Conducting Motor-Starting Studies and Analysis of Industrial and Commercial Power Systems (motor starting current and voltage drop).
- Manufacturer example: residential single-phase standby generator spec sheets, such as the Generac Guardian series, list power factor 1.0.
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