kVA to kW Calculator
Translate the kVA on a generator, UPS or transformer nameplate into the real power it can actually deliver — the difference between what a machine is rated at and what a load can take out of it.
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kVA to kW Calculator
Updates as you type- Not available as real powerNameplate kVA minus deliverable kW
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- Share of the nameplate you can use
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- Reactive power√(kVA² − kW²). Not the same thing as the shortfall above
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kW = kVA × PF · No voltage or phase required — power factor belongs to the load, not the supply.
How this is calculated
Every substantial piece of electrical plant carries two power figures, and they are not alternatives. The kVA rating is apparent power: volts times amps, the total the machine can pass. The kW rating is real power: the portion that arrives at the load as heat, light or shaft work. The ratio between them is the power factor, and that single number is the whole of this conversion.
kW = kVA × PF
Notice what the formula does not contain. There is no voltage and no phase, and neither is missing by accident — the relationship between apparent and real power is a property of the load, not of the supply feeding it. A 100 kVA set at 0.8 delivers 80 kW whether it is wired for 208 V three-phase or 240 V single. That independence is what separates this page from the current converters, which need both and use no power factor at all.
Generator sets are conventionally rated at 0.8 lagging, which is why the two numbers on a nameplate almost always sit in a 5:4 ratio. It is a standard assumption about the mix of motor and resistive load a set is expected to see, not a measurement of any particular installation. The converter defaults to it for that reason, and a machine that states its own kW figure overrides anything derived here.
The capacity that does not appear as kW has not gone anywhere. It isreactive power, measured in kVAR, cycling into the magnetic fields of motors and transformers and back out again a hundred times a second. It does no work and it is not billed as energy, but it occupies conductor and winding capacity exactly as working current does. That is why the shortfall is real: 20 kVA of a 100 kVA machine is genuinely unavailable for useful output at 0.8, even though nothing is being wasted in the ordinary sense.
One warning about arithmetic. The shortfall and the reactive power are different quantities. Subtracting kW from kVA gives the first; the second is the third side of a right triangle, and at 10 kVA and 0.8 those come out as 2 and 6 respectively.
Worked example
One full calculation with real numbers, so you can follow along and check the tool by hand.
Start with the case people actually hit: a 10 kVA UPS. The unit is advertised, boxed and quoted as 10 kVA. Its specification sheet, further down, gives a power factor of 0.8. So the real output is 10 × 0.8 = 8 kW, and the equipment plugged into it may draw 8,000 W — not the 10,000 W the headline figure suggests.
This is not a trick and the unit is not mis-sold. Its output stage was designed to supply 10 kVA to a load at 0.8, which is what a mixed load looked like when that product line was specified. Plug a rack of modern servers into it instead — all running at 0.95 or better — and you will exhaust the 8 kW long before you get anywhere near the 10 kVA. The kW figure is the binding one, and it is the one to compare your load against.
Now scale it up, where the same arithmetic has a twist. A 100 kVA standby generator at the same 0.8 is an 80 kW machine. Suppose the site turns out to be almost entirely resistive heating, running at a power factor of 1.0. Does the set now deliver 100 kW?
It does not, and the reason is worth knowing. A generator has two limits in series. The alternator is limited by current, so its ceiling is 100 kVA. The diesel engine turning it is limited by torque, and its ceiling is 80 kW, which is what its rating was chosen to match. At unity power factor the alternator is loafing — 80 kW is only 80 kVA — but the engine is flat out. Improving power factor lets youreach the kW rating; it never lets you exceed it.
A transformer behaves differently again, and the contrast is the useful part. It has no engine, so it has one limit: the thermal kVA rating. It will pass 100 kVA at any power factor at all, and the kW figure is only ever a statement about how much of that arrives as work.
Visual comparison
| Power factor | Bar | Real output (kW) | Capacity not delivered |
|---|---|---|---|
| PF 0.60 | 6 | 4 kVA unusable | |
| PF 0.65 | 6.5 | 3.5 kVA unusable | |
| PF 0.70 | 7 | 3 kVA unusable | |
| PF 0.75 | 7.5 | 2.5 kVA unusable | |
| PF 0.80 | 8 | 2 kVA unusable | |
| PF 0.85 | 8.5 | 1.5 kVA unusable | |
| PF 0.90 | 9 | 1 kVA unusable | |
| PF 0.95 | 9.5 | 0.5 kVA unusable | |
| PF 1.00 | 10 | 0 kVA unusable |
Source: A fixed 10 kVA rating, the worked example above, evaluated across the range.
Reference tables
The converter needs a power factor and most readers do not have a measured one. This table is for that case: find the class of load that dominates your installation and take the typical column, which is a defensible figure to design around rather than a guess.
Read the range as well as the typical value. A wide range means the class tells you very little on its own — LED lighting spans 0.9 to 0.95 depending entirely on driver quality, and computer equipment reaches 0.99 with active correction while older supplies without it fall much further. A narrow range means the class is genuinely predictable, which is why resistive heating gets a single number.
Two rows are worth reading against each other. An induction motor at full load sits around 0.85; the same motor lightly loaded falls to 0.6, because its magnetising current barely changes with the work it is doing. Oversized motors idling are the single largest reason real sites underperform their calculated power factor, and no class average will catch it.
Where the equipment states its own figure, use that instead of anything here. A nameplate ratio is a measurement of a specific design; this table is a description of a category.
| Class of load | Usual range | Use this | What moves it |
|---|---|---|---|
| Resistive heating, incandescent lamps | 1.00 | 1.00 | A resistor has no reactive component at all. This is the one row that is exact rather than typical. |
| Electric range, oven, water heater | 0.98 – 1.00 | 1.00 | Elements are resistive; only the small control and fan loads pull the figure fractionally below unity. |
| LED and electronic lighting | 0.90 – 0.95 | 0.92 | Driver quality decides it. Commercial fixtures are usually corrected above 0.9; cheap retrofit lamps can be far worse. |
| Computers and IT equipment | 0.90 – 0.99 | 0.95 | Active power factor correction has been standard in PC and server supplies for years, so modern equipment sits near unity. |
| Induction motors at full load | 0.80 – 0.90 | 0.85 | The classic reactive load. Larger motors correct better than small ones — a 100 hp machine beats a 5 hp machine. |
| Induction motors, lightly loaded | 0.50 – 0.70 | 0.60 | Magnetising current is roughly constant whatever the shaft is doing, so an oversized motor idling is the worst case on any site. |
| Fluorescent with magnetic ballast | 0.50 – 0.60 | 0.55 | Uncorrected magnetic ballasts are poor. Fixtures sold as "high power factor" carry a correction capacitor and reach 0.9. |
| Welders and arc equipment | 0.50 – 0.70 | 0.60 | Also intermittent, so the duty cycle matters as much as the power factor when sizing anything that feeds one. |
| Older UPS units, standby topology | 0.60 – 0.70 | 0.65 | The reason a 1,000 VA unit of this generation is a 600 W unit. The ratio is the design power factor of the output stage. |
| Modern UPS units, online or lithium | 0.90 – 1.00 | 0.95 | Newer units are increasingly rated 1:1, so the VA and watt figures on the box are the same number. |
| Generator sets, nameplate assumption | 0.80 | 0.80 | Not a measurement but a convention: sets are rated at 0.8 lagging by standard, which is why kW is four fifths of kVA on the plate. |
Common mistakes
Assuming a kVA rating is deliverable wattage
The confusion this page exists to settle. A 100 kVA generator is not a 100,000 W generator — it is an 80,000 W generator, because sets are rated at 0.8. Budgeting for the larger number is how a standby installation ends up one size short before anyone has switched anything on.
Treating the shortfall as waste or as a defect
The gap is reactive power, and it is neither lost nor billed as energy. It circulates between the supply and the load twice a cycle, doing no work but occupying capacity. The machine is not underperforming; apparent power is simply the larger of the two quantities.
Reading the shortfall as kVAR
They are close together and they are not the same. At 10 kVA and 0.8 the shortfall is 2 kVA of rating, while the reactive power is 6 kVAR — the sides of a right triangle do not subtract linearly. Order a capacitor bank from the wrong one and it will be a third of the size you needed.
Applying the conversion to a transformer as if it capped output
A transformer has one kVA rating and it is thermal. It will pass its rated kVA at any power factor; the kW figure just tells you how much of that arrives as work. A generator is different, because its engine imposes a second, separate kW limit.
Using this to size conductors
Nothing here gives a current. Cable and protection are sized from amps, which come from the apparent power and the voltage — not from the real power and certainly not from a figure that has already had power factor applied to it.
This tool provides planning estimates. Always verify final values against your local code and a licensed electrician.
Frequently asked questions
How many kW is a 10 kVA UPS?
At the common 0.8 rating, 8 kW. That is why a unit sold as "10 kVA" so often turns out to deliver 8,000 W and no more: the two figures are printed side by side on the specification sheet and the larger one gets used in the marketing. Newer online units are increasingly rated 1:1, so a 10 kVA unit of that generation genuinely is a 10 kW unit — check the plate rather than assuming either way.
Why are generators rated in kVA rather than kW?
Because the alternator is limited by current, and current is set by apparent power regardless of how much work it does. Windings heat according to the amps passing through them whatever the phase relationship. The engine behind the alternator, meanwhile, is limited by torque and is rated in kW. A set therefore carries two ceilings, and which one you hit first depends on the power factor of what you plug in.
Can I get more than the rated kW out of a set if my load is resistive?
No, and this is the practical consequence of those two ceilings. A 100 kVA / 80 kW set feeding a purely resistive load at unity power factor still stops at 80 kW, because the engine cannot produce more mechanical power than it is rated for. The alternator would happily pass more current; the engine will not turn it. What a good power factor buys you is the ability to reach the kW rating, not to exceed it.
What power factor should I use if I do not know mine?
For a mixed commercial site, 0.85 is a defensible working figure. For an industrial site with a lot of motor load, 0.8. For a data centre or anything dominated by modern IT equipment, 0.95 or better. If you are checking a nameplate rather than a site, use whatever ratio the machine itself states — divide its kW figure by its kVA figure and you have the exact number the manufacturer designed to.
Is kW the same as the kWh on my bill?
No, though they are related. Kilowatts are a rate — how fast energy is being used at this instant. Kilowatt-hours are a quantity — that rate multiplied by time. A 10 kW load running for three hours consumes 30 kWh. Most tariffs bill energy in kWh, and many commercial tariffs also bill a separate demand charge based on the highest kW or kVA reached during the period, which is where power factor starts costing real money.
How do I get the current instead?
That is a different calculation and it needs two inputs this page does not ask for: a voltage and a phase. It also applies no power factor at all, because current is carried by apparent power. Use the kVA to amps converter, feed it the same nameplate rating and the winding voltage, and you will get the figure conductors and protection are sized from.