kW to kVA Calculator
You know the load in kW. The transformer, the generator and the UPS are all sold in kVA. This is the conversion between what the load does and what the source has to be rated for — and it is where sizing decisions get made.
Inputs
| Project | Circuit ref | ||
| Prepared by | Date | ||
| Checked by | Date |
kW to kVA Calculator · S = P / PF · EnergyCalcHQ · energycalchq.com
Preliminary calculation. The figures behind it are representative values for the stated conditions, not a substitute for the current edition of the standard or the manufacturer's published data. Verify before issuing for construction. Not a substitute for a qualified engineer or a protection study.
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The formula
S = P / PF
Where P is real power in kW, PF is power factor, and S is apparent power
in kVA.
Dividing by a number below 1 always makes the answer bigger. That is the point: the source has to be rated for the current it supplies, and reactive current flows whether or not it does work.
A worked example
A factory with a 45 kW connected load running at 0.86 power factor:
S = 45 / 0.86 = 52.3 kVA
So the supply has to be good for 52.3 kVA, even though only 45 kW does anything useful. The extra 7.3 kVA is the magnetising current the motors need to establish their fields — necessary, but not work.
How much the power factor costs you
Take a fixed 100 kW load and watch what the source has to be rated for:
| Load PF | kVA required | Extra capacity over 100 kW |
|---|---|---|
| 1.0 | 100 kVA | — |
| 0.95 | 105 kVA | 5 % |
| 0.9 | 111 kVA | 11 % |
| 0.85 | 118 kVA | 18 % |
| 0.8 | 125 kVA | 25 % |
| 0.75 | 133 kVA | 33 % |
| 0.7 | 143 kVA | 43 % |
| 0.6 | 167 kVA | 67 % |
| 0.5 | 200 kVA | 100 % |
At 0.7 power factor you are buying, installing and paying maximum demand charges on a transformer 43 % larger than the work you are doing requires. At 0.5 you are buying twice the transformer.
This is the entire commercial case for power factor correction. Capacitors are cheap compared with a transformer one frame size larger, a cable one size larger, and a monthly maximum demand charge levied in kVA — which most Indian industrial tariffs are. The kVAr converter sizes the correction; APFC panel step sizing covers building the panel that switches it.
kW to kVA chart
| kW | PF 0.7 | PF 0.8 | PF 0.85 | PF 0.9 | PF 0.95 |
|---|---|---|---|---|---|
| 5 | 7.1 | 6.3 | 5.9 | 5.6 | 5.3 |
| 10 | 14.3 | 12.5 | 11.8 | 11.1 | 10.5 |
| 15 | 21.4 | 18.8 | 17.6 | 16.7 | 15.8 |
| 20 | 28.6 | 25.0 | 23.5 | 22.2 | 21.1 |
| 30 | 42.9 | 37.5 | 35.3 | 33.3 | 31.6 |
| 45 | 64.3 | 56.3 | 52.9 | 50.0 | 47.4 |
| 50 | 71.4 | 62.5 | 58.8 | 55.6 | 52.6 |
| 75 | 107 | 93.8 | 88.2 | 83.3 | 78.9 |
| 100 | 143 | 125 | 118 | 111 | 105 |
| 125 | 179 | 156 | 147 | 139 | 132 |
| 150 | 214 | 188 | 176 | 167 | 158 |
| 200 | 286 | 250 | 235 | 222 | 211 |
| 250 | 357 | 313 | 294 | 278 | 263 |
| 300 | 429 | 375 | 353 | 333 | 316 |
| 400 | 571 | 500 | 471 | 444 | 421 |
| 500 | 714 | 625 | 588 | 556 | 526 |
From kVA to a standard rating
The calculated kVA is a minimum, not a purchase order. Three things happen between the two.
Round up to a standard rating. Transformers come in 25, 63, 100, 160, 250, 315, 400, 500, 630, 1000, 1600 kVA. Our 52.3 kVA lands on a 63 kVA unit.
Add headroom for growth. A transformer sized exactly to today's load has no room for the extension nobody has mentioned yet. 20 % is a common allowance, which would push the same example toward 63 kVA comfortably, or 100 kVA if expansion is genuinely expected.
Apply diversity going the other way. Connected load is not maximum demand — nothing runs everything simultaneously. If your 45 kW is the sum of every nameplate on site, the actual peak is lower, often much lower. See connected load and maximum demand for the factors.
These pull in opposite directions, which is why sizing is judgement rather than arithmetic. The arithmetic just tells you where to start.
Loading a transformer: what to aim for
A transformer is most efficient somewhere near 50 % of its rating, where copper losses (which rise with the square of load) and iron losses (which are constant) balance.
| Loading | Verdict |
|---|---|
| Below 30 % | Iron losses dominate; you are paying for magnetising an oversized core all day |
| 40 – 70 % | The sweet spot — efficient, with room for growth and for motor starts |
| 70 – 85 % | Acceptable, running warm, no room for expansion |
| Above 85 % | Winding temperature and life become the concern |
| Above 100 % | Insulation life halves for roughly every 6 – 8 °C over rated temperature rise |
So a 52.3 kVA load on a 100 kVA transformer is not oversizing — it is 52 % loading, right in the efficient band.
Sizing a UPS or a generator from kW
UPS. Same conversion, and then a second one. Convert the equipment's watts to VA at the UPS's power factor — which is stated on its nameplate, not on your load's. Then leave headroom: a UPS at 100 % load runs hot, and heat is what kills batteries. See watts to VA.
Generator. Convert to kVA, then check the starting case separately. A DG set that comfortably carries the running kVA can still stall or dip badly when a large motor starts direct-on-line, because starting kVA is three to five times running kVA. The DG set sizing calculator works through both.
Common mistakes
Multiplying instead of dividing. kW to kVA divides by power factor; kVA to kW multiplies. Getting it backwards on a 0.8 PF load gives 64 kVA for a 80 kW load instead of 100 kVA — a 36 % undersize.
Sizing on connected load with no diversity. Adds up to a transformer far larger than the site will ever draw.
Ignoring future load. The cheapest time to install a larger transformer is the first time.
Using the load's power factor for a UPS. The UPS nameplate power factor is the one that governs its own rating.
The two ratings a UPS carries, and why they disagree
A UPS is advertised with a VA figure and a watt figure, and the ratio between them is the manufacturer's assumed power factor rather than anything about your load. Older units assume 0.6 or 0.7; most current designs assume 0.9, and some are rated at unity.
That assumption is a limit, not a conversion. The unit will refuse whichever figure it reaches first. A 3000 VA / 2700 W machine loaded with 2800 W of corrected electronic supplies is over its watt rating while sitting at 93 per cent of its VA rating, and it will alarm and transfer to bypass on a limit the VA number gave no warning of.
The practical rule is to check the load against both figures independently and size on whichever is tighter. It is also worth noting that the watt rating is the one that governs runtime, because the battery is delivering real power — two UPS units with identical VA ratings and different watt ratings will not give the same backup time on the same load.
Generators are rated the same way, and derated differently
An alternator is limited by winding current, so its rating is in kVA for exactly the reason a transformer's is. The engine behind it is limited by torque, so its capability is in kW. A generating set carries both, related by a power factor the manufacturer has assumed — almost universally 0.8 lagging.
Feed that set a load at 0.95 power factor and the engine becomes the limit before the alternator does: the set can deliver its full kVA only if the load is reactive enough to keep the real power within the engine's capability. A 125 kVA / 100 kW set supplying a 0.95 power factor load can produce about 105 kVA before the engine reaches 100 kW, and the remaining alternator capacity is unusable.
That is the opposite of the intuition most people bring to power factor correction, and it catches sites that install capacitors and then run on generator during an outage. Correcting to unity in front of a generating set does not release capacity — it removes the reactive load the set was rated against, and on some machines a leading power factor will destabilise the automatic voltage regulator entirely. Where a plant has both capacitors and a generator, the correction stages should be interlocked to drop out when the set is running.
Every conversion on this site runs in your browser — nothing you type is sent anywhere. See all 11 calculators.
Questions people ask
- How much extra source capacity does poor power factor cost?
- For a fixed 100 kW load: 100 kVA at unity, 105 kVA at 0.95, 111 kVA at 0.9, 118 kVA at 0.85 and 125 kVA at 0.8. So a plant running at 0.8 has to buy a quarter more transformer, cable and switchgear than one running at unity for exactly the same useful work — which is the whole commercial case for power factor correction, before any tariff penalty is counted.
- Why are transformers, gensets and UPS units sold in kVA?
- Because their limit is heating, heating follows current, and current follows kVA regardless of what the load's power factor happens to be. The manufacturer has no idea what you will connect and does not need to. Your load is measured in kW because that is the work being done; the source is rated in kVA because that is the current it has to carry.
- Does this conversion need the voltage or the √3?
- No. S = P / PF, and that is the whole of it — no voltage, no phase count, no √3. Those matter for current, not for this. kVA and kW are both power, and power factor is simply what fraction of the apparent power does work. If you need the current as well, convert the kVA afterwards.
- Should I add a margin on top of the kVA figure?
- Yes, but decide which margin you are adding. A transformer wants to sit in the 60 to 80 per cent loading band — below 60 its iron losses run around the clock for nothing, above 80 you are planning the next one. The more useful question is whether the margin is really needed: correcting the power factor first reduces the kVA figure itself, and on a plant at 0.8 that is a quarter of the rating recovered without buying a larger unit.