The Catalog Sells kVA, Your Rack Draws kW
Open any UPS or generator datasheet and the model number carries its kVA: the APC Smart-UPS 1500VA, the Generac 22kW (which is actually a 27.5 kVA machine), the data-center 800 kVA transformer. The equipment you are protecting, meanwhile, is quoted in watts on every PSU sticker. Bridging the two is the power factor of the load — and for a rack of servers with active PFC supplies it is 0.95 to 0.99, while for motor loads it sinks toward 0.7. This converter keeps the two straight: divide the watts by the PF to find the kVA you must buy.
kVA = kW ÷ power factor
S = P / cos φ
At 0.8 PF every kW costs 1.25 kVA of rating; at 1.0 the two are equal.
Typical Sizing Decisions
| load (kW) | PF | kVA needed | Purchase decision |
|---|---|---|---|
| 1.2 kW | 0.8 | 1.5 kVA | Home office — the 1500 VA UPS. |
| 5 kW | 0.8 | 6.25 kVA | Small server rack — buy 8 kVA, not 6 kVA. |
| 5 kW | 0.99 | 5.05 kVA | Same rack with PFC power supplies — 6 kVA suffices. |
| 40 kW | 0.8 | 50 kVA | Telco or office floor — 50 kVA transformer. |
| 90 kW | 0.9 | 100 kVA | PFC-corrected floor — a 100 kVA unit, fully utilized. |
The 5 kW pair is the practical takeaway: identical load, two power factors, and the difference decides between the 6 kVA and 8 kVA price tier. That is why modern server PSUs advertise "active PFC" — 0.99 power factor means a 5 kW rack needs barely more kVA than kW, shrinking the UPS, the generator, and the branch circuit all at once. The same arithmetic in reverse, converting a nameplate kVA back to usable kW, is the kVA to kW page.
Engineering Context
The mirror calculation — what a nameplate kVA delivers as kW — is kVA to kW, and the underlying ratio is explained on the power factor page. When the load is a motor rather than a rack, its rating arrives in horsepower and must be converted before it enters this formula. The branch circuit and breaker that feed the UPS are sized by the current, not the kVA — see watts to amps at your supply voltage. Everything traces to the electric hub.
More: kVA to kW · Power Factor · W to A · Electric Hub
Related Unit Converters
Frequently Asked Questions
How do I convert kW to kVA?
Divide by the power factor: kVA = kW ÷ PF. A 5 kW server rack with a 0.8 power factor needs 5 ÷ 0.8 = 6.25 kVA, so you buy the 8 kVA UPS model. The kW is what the load does; the kVA is the current-handling the equipment must provide.
How do I size a UPS for my equipment?
Add up the nameplate watts of everything to be protected, convert that total kW to kVA at the UPS's rated power factor (0.8 for most online double-conversion units), then pick the next standard size up. A 5 kW total therefore lands on an 8 kVA UPS — 6.25 kVA calculated, 8 kVA purchased — leaving headroom for inrush and future loads. Never size a UPS at 100% of its rating; 80% utilization is the practical ceiling.
Why does the power factor change the size I must buy?
Because the UPS's kVA rating caps its output current, and real load current carries both the working current (kW) and the reactive sloshing current that does no work. At PF 0.8, every kW of real load drags along 1.25 kVA worth of current — so the machine has to be rated 25% larger than the kW alone would suggest. Higher-PF loads (PFC power supplies at 0.95–0.99) shrink that gap.