kW vs kVA: Power Factor in Plain Terms
Last reviewed 2026-11-10
Look at the nameplate of a generator, a transformer or a UPS and the headline rating is in kVA. Look at the loads it feeds, such as heaters, motors and lighting, and the ratings are usually in kW. Treating the two as interchangeable is one of the most common mistakes in early-stage electrical design. Usually it shows up as an undersized generator or a cable that runs hotter than expected. This guide explains what each unit measures, why both exist, and how power factor links them.
Two kinds of current, one wire
In an AC circuit, voltage and current both oscillate as sine waves at the supply frequency. With a purely resistive load such as an electric heater, the two waves rise and fall together. Every moment that current flows, it flows with the voltage behind it, and all of it delivers energy to the load.
Motors, transformers, fluorescent ballasts and anything else with a magnetic circuit behave differently. Part of the current they draw builds up a magnetic field during one quarter of the cycle and hands that energy back during the next. Averaged over a cycle, that part of the current does no net work. It still flows through every cable, switch and winding between the source and the load, and it still heats them.
The result is a current wave that lags the voltage wave by some angle, called φ (phi). The bigger the angle, the larger the share of current that is just shuttling energy back and forth.
The three powers
Engineers split the power flowing in such a circuit into three quantities:
- Real (active) power, P, in kW. The rate at which energy is actually converted into heat, light or mechanical work. Your energy meter adds it up as kWh.
- Reactive power, Q, in kVAr. The rate at which energy oscillates between the source and the magnetic or electric fields of the load. It does no net work, but it needs current.
- Apparent power, S, in kVA. Simply voltage times current (times √3 for three-phase). It tells you how much current the equipment has to carry, whatever that current is doing.
The three are related like the sides of a right-angled triangle, the power triangle:
Real power lies along the base, reactive power stands upright, and apparent power is the hypotenuse. The angle between P and S is the same φ as the angle between the voltage and current waves.
Power factor is the ratio
Power factor (PF) is the ratio of real power to apparent power: PF = P / S = cos φ. It runs from 0 to 1. A heater has a PF close to 1. A large induction motor at full load might sit around 0.85 to 0.9, and the same motor at a quarter load can drop well below 0.7, because its magnetising current stays almost the same while the useful current falls.
A worked example makes it concrete. Take a 1000 kVA generator operating at PF 0.8:
The machine is carrying its full rated current, but only 800 kW of that is turning into useful output. The other 600 kVAr is the magnetising requirement of the connected loads. You can try other values in the kVA to kW converter.
Strictly, cos φ is the displacement power factor. Electronic loads such as variable-speed drives, rectifiers and LED drivers also distort the current waveform with harmonics, which lowers the true power factor below cos φ. A power quality analyser measures true PF. For many traditional motor and lighting loads the two are close, but for electronic loads the difference can matter.
Why sources are rated in kVA
What limits a transformer, an alternator or a cable is heat, and heat comes from current. A transformer winding gets just as hot carrying 1000 A at PF 0.6 as at PF 1.0. The I²R loss is identical, even though the first case delivers far less useful power. So the manufacturer cannot promise a kW output without knowing your power factor, but can promise a kVA output. That is why transformers and UPS units carry kVA ratings, and why their kW capability depends on the load.
Generator sets are a special case because they have two limits. The engine produces mechanical power, so it limits kW. The alternator carries current, so it limits kVA. Most sets are rated in kVA at a standard power factor of 0.8, which fixes the kW rating at 80 % of the kVA figure. A load at PF 0.95 will run into the engine's kW limit before the alternator is fully used. A load at PF 0.6 will hit the alternator's current limit while the engine still has spare power. Proper sizing checks both, which is what the generator sizing calculator does.
From kVA to amps
Because kVA is just volts times amps, current follows directly:
If you start from kW, you have to go through the power factor first: I = P / (√3 × V_LL × PF). A 100 kW load at 400 V draws 144.3 A at unity power factor but 180.4 A at PF 0.8, which is 25 % more current for the same useful power. That extra current is what sizes the cable, the breaker and the transformer. The line current calculator shows the full working.
Common mistakes
Adding kVA from loads with different power factors
kVA values from loads at different power factors do not add arithmetically, because they point in different directions on the power triangle. The correct method is to add all the kW, add all the kVAr, and combine them at the end: S = √(ΣP² + ΣQ²). Adding kVA directly overstates the total, which errs on the safe side but can lead to an oversized and more expensive source.
Assuming kW and kVA are the same "because PF is roughly 1"
Modern equipment with active power-factor correction often runs close to unity, but motor-heavy sites, older lighting and lightly loaded transformers do not. If the number matters, measure it.
Forgetting efficiency
A motor nameplate gives the mechanical output at the shaft. The electrical input is larger by the motor's efficiency. A 15 kW motor at 92 % efficiency takes about 16.3 kW from the supply, and at PF 0.85 that is about 19.2 kVA. Both conversions have to be made.
What to do about low power factor
A low power factor means more current than the useful power requires. That means bigger cables and transformers, higher losses, more voltage drop and, under many tariffs, a reactive-energy or kVA demand charge. The usual remedy is to supply the reactive power locally with capacitors, so it no longer has to travel through the network. The capacitor kVAr needed to move from one power factor to another is Q = P × (tan φ1 − tan φ2). Raising a 100 kW load from PF 0.70 to 0.95 needs about 69 kVAr. The power factor correction calculator works it out, along with the capacitance per phase and the current saved.
Correction has its own risks. Capacitors combined with the inductance of the supply form a resonant circuit, and on sites with significant harmonic distortion that resonance can amplify harmonics enough to damage equipment. Detuned banks with series reactors and a harmonic study are the normal precautions where drives and rectifiers make up a large share of the load.
The short version
- kW is the useful power and the figure you pay for as energy. kVA is the volts-times-amps that equipment must carry.
- Power factor is kW ÷ kVA. It is 1 for resistive loads and lower for motors and magnetic equipment.
- Transformers, UPS units and alternators are rated in kVA because current, not useful power, makes them hot. Generator engines are limited in kW.
- Add loads as kW and kVAr separately, then combine. Never add kVA from loads with different power factors directly.
- Low PF costs money in larger equipment and tariff charges. Capacitors fix it, with care around harmonics.
This guide is general education. Equipment ratings and tariff rules vary, so confirm against manufacturer data and your utility's terms.