Convert kVA to kW and kW to kVA

Why UPS capacity is in kVA when servers are in watts

This is one of those mismatches that causes real ordering mistakes, so it is worth understanding rather than memorizing.

Your servers are rated in watts because watts describe what they consume and turn into work and heat. Your electrical infrastructure — the conductors, the breakers, the transformer, the UPS — is limited by current, not by useful work. And current tracks apparent power, which is measured in kVA. So the two sides of the same system get described in two different units.

Power factor is the bridge. It is the fraction of the apparent power that actually does something useful. At a power factor of 0.95, 95% of what the system carries does work and the remaining 5% sloshes back and forth without accomplishing anything — but your conductors still have to be sized to carry it.

The practical upshot: a 10 kW IT load needs more than 10 kVA of UPS. At 0.95 it needs 10.53 kVA, and that is before you add headroom or redundancy.

Which power factor to actually use

For anything modern, 0.95 is a safe default and 0.99 is achievable. The 80 PLUS certification that essentially every server power supply carries requires 0.9 or better once the supply is at half load, and measured units with active power factor correction usually land between 0.95 and 0.99.

You will still see 0.8 quoted in older references and the occasional consultant's spreadsheet. That number comes from the era of unregulated linear power supplies and does not describe equipment you can buy today. Using it will oversize your UPS and your circuits by about 19% — not dangerous, but expensive, and it can push you into a larger UPS frame or an extra circuit you did not need.

If you have metered PDU data, use the measured power factor rather than any of these. Real numbers beat rules of thumb every time.

The 80% rule, and which way to apply it

Once you have a kVA figure, the next question is what circuit carries it — and this is where a lot of people get the arithmetic backwards.

The rule itself is straightforward. NEC treats any load running three hours or more as continuous, which every production rack is. Continuous loads are capped at 80% of the breaker rating. So a 30 A circuit is not a 30 A circuit for your purposes; it is a 24 A circuit with 6 A you are not allowed to touch.

The trap is the direction. If someone hands you a circuit rating, multiply by 0.8 to find what you can load onto it. If you are starting from your load, you divide by 0.8, and the circuit you need comes out larger than the load — not smaller. Applying the derate to a load figure and shopping for a smaller circuit is how racks end up on breakers that trip under sustained draw.

The panel above the explanation does both directions at once so you do not have to remember which is which, and it names the smallest standard receptacle that will actually carry the load.

The nameplate trap that catches people

Look at a UPS spec sheet and you will often see two numbers: 10 kVA / 8 kW, for example. It is easy to read that 0.8 as a statement about typical loads. It is not. It is a limit built into that specific UPS — the most real power it can deliver, regardless of what you plug into it.

So a 10 kVA / 8 kW unit will give you 8 kW and not a watt more, even if your servers run at a beautiful 0.99 power factor and you calculated that you should be able to draw 9.9. The unit caps you first.

When you size, check both numbers on the nameplate and design against whichever one runs out first. Buying a 10 kVA UPS for a 9 kW load looks fine on the kVA line and leaves you 1 kW short in practice.

The good news is this is fading. Newer units increasingly ship at unity power factor — rated 10 kVA / 10 kW — which removes the trap entirely. Worth checking, because it can change which model you buy.