kW vs. kVA Explained

kW (kilowatts) is real power: the part of the electrical supply that does work, turning a motor shaft, heating a resistor or running a server. kVA (kilovolt-amperes) is apparent power: the voltage multiplied by the current actually flowing, whether or not all of that current is doing work. The ratio between them is the power factor.

At a power factor of 1.0 the two are the same number. Below 1.0, kVA is always the larger one, because some of the current is flowing back and forth to set up magnetic and electric fields (reactive power, measured in kVAR) rather than delivering energy to the load. That current still has to be carried by every cable, breaker and transformer winding in the path.

The power triangle

Real power (kW) and reactive power (kVAR) are at right angles to each other, and apparent power (kVA) is the hypotenuse:

The angle φ is the phase shift between voltage and current. Inductive loads such as motors and transformers draw current that lags the voltage. Capacitive loads draw current that leads it. Either way, the further φ is from zero, the more kVA it takes to deliver the same kW.

How much kVA per 100 kW

Every figure in the table is arithmetic from the two formulas above: kVA = 100 ÷ PF, and kVAR = √(kVA² − 100²). Going from 0.95 to 0.80 raises the apparent power by about 19% for exactly the same useful work.

Converting kW to kVA and amps: a worked example

A 400 kW load at 0.90 power factor, fed at 480 V three-phase:

• Apparent power: kVA = 400 ÷ 0.90 = 444.4 kVA
• Reactive power: kVAR = √(444.4² − 400²) = 193.7 kVAR
• Line current: I = kVA × 1000 ÷ (1.732 × V) = 444,400 ÷ (1.732 × 480) = 535 A

Drop the power factor to 0.80 and the same 400 kW becomes 500 kVA and 601 A: 66 more amps through every conductor, for no extra work done. For single-phase circuits, leave out the 1.732: I = kVA × 1000 ÷ V.

Why equipment is rated in kVA

A transformer or alternator does not know what the load is doing with the current. Its winding losses, and therefore its heating, scale with the current squared (I²R), and its core losses scale with voltage. Both depend on volts and amps, not on the phase angle between them. So the limit is stated in volt-amperes, and the equipment can deliver its full rating in kW only when the load's power factor is 1.0.

Loads, on the other hand, are described by the work they do, and energy is billed in kWh. That is why the conversion has to happen at the point where a load list meets an equipment rating.

Transformers

Size on kVA: divide the load in kW by the expected power factor, then go up to the next standard nameplate from ANSI/IEEE C57.12.00. The 444 kVA example above lands on a 500 kVA unit. The Transformer Sizing Calculator does this step and returns full-load amps. Reading the result back off a real unit is covered in How to Read a Transformer Nameplate.

Generators

Generator sets carry both numbers because they have two limits. The engine limits kW, since real power is what burns fuel. The alternator limits kVA, since that sets winding current. By industry convention, three-phase gensets are rated in kW at 0.8 power factor, so a 2,000 kW set is a 2,500 kVA set. A load at 0.9 power factor can use the full kW rating. A load at 0.7 hits the alternator's kVA limit before the engine's kW limit. Generators also tolerate leading (capacitive) power factor poorly, so a lightly loaded site with large filter capacitors can have voltage-regulation problems even though it is well inside the kW rating. Fuel burn tracks kW, which is what the Generator Runtime Calculator works from. The full sizing method is in How to Size a Standby Generator.

UPS systems

A UPS has a kVA rating and a kW rating, and either one can be the binding limit. Older three-phase designs were commonly rated at 0.8 output power factor, so a 500 kVA unit could only deliver 400 kW. Many current designs are rated at 0.9 or 1.0. Modern server power supplies with active power-factor correction typically run close to unity, so for IT load the kW rating is usually the one that runs out first. Always check the kW figure, not just the kVA on the front of the datasheet. The full method, including redundancy and battery runtime, is in How to Size a UPS.

Where power factor costs money

Beyond equipment sizing, low power factor shows up on the utility bill. Some tariffs bill demand in kVA rather than kW, and others add a charge when power factor falls below a threshold set in the tariff. The fix is usually local correction: capacitor banks, or active filters where harmonics are present, that supply the reactive power on site so it is no longer drawn through the utility service. The Power Factor Correction Calculator sizes the kVAR needed to reach a target power factor and shows how much apparent power is released.

Common mistakes

• Sizing a transformer on kW. A 400 kW load at 0.85 needs about 471 kVA, not 400.
• Applying power factor twice. A genset's kW rating already assumes 0.8 power factor. Converting the load to kVA and then comparing it to the kW rating double-counts.
• Checking only one UPS rating. A load can fit the kVA rating and still exceed the kW rating, or the other way round.
• Using the three-phase formula on a single-phase circuit. Dividing by 1.732 when you shouldn't under-states current by about 42%.
• Correcting to exactly 1.0. As load drops, a fixed capacitor bank sized for unity will push the system into a leading power factor. Targets of 0.95 to 0.98 are the norm.

Frequently asked questions