Start with the load in kW
The most reliable way to size a UPS is to add up the real power (kW) of everything you want to protect — server nameplate ratings, switch power consumption, monitor loads and so on. If you only have current ratings, single-phase power (kW) ≈ volts × amps × power factor ÷ 1000, and three-phase ≈ √3 × volts × amps × power factor ÷ 1000.
kVA vs kW and power factor
Apparent power (kVA) is what the UPS is rated in; real power (kW) is what your load actually consumes. The two are linked by power factor (kW = kVA × pf). Older loads had a power factor of 0.7–0.8, so a 10 kVA UPS only delivered 7–8 kW. Modern unity power factor (pf 1) UPS — like Sentryum, Sentinel and Multi Power — make kVA equal kW, so a 10 kVA unit gives you a full 10 kW. Always check whether your load figure is kW or kVA before selecting a UPS.
Add headroom and inrush
Never run a UPS at 100%. Allow 20–25% headroom for future growth and to keep the UPS running efficiently (online UPS are most efficient at 50–80% load). Then consider inrush: motors, compressors, blade servers and power supplies can draw 2–10× their running current for a few cycles at switch-on. An oversized or overload-rated UPS (Sentryum handles 150% for 1 minute) absorbs this without transferring to bypass.
Runtime and battery sizing
Sizing the UPS for power is only half the job — you also size the battery for runtime. Decide how many minutes of autonomy you need (enough for a graceful shutdown, a generator start, or a full operating window), then add battery strings or external cabinets to meet it. Use our runtime calculator to estimate, or the UPS Finder for an engineer-led recommendation.
Frequently asked questions
What is the difference between kVA and kW on a UPS?
kVA is apparent power (the UPS rating); kW is real power (what your load uses). They are linked by power factor. A unity pf (pf 1) UPS delivers kW equal to its kVA rating.
How much headroom should I allow on a UPS?
Size the UPS to run at 75–80% of capacity, leaving 20–25% headroom for growth, inrush and efficient operation.
How do I convert amps to kVA?
Single-phase: kVA = (V × A) ÷ 1000. Three-phase: kVA = (√3 × V × A) ÷ 1000. Divide by power factor to get kW if the load pf is not 1.
Can a UPS be too big?
Yes. An oversized UPS runs at very low load, which can reduce efficiency and waste energy. Right-sizing to 50–80% load is ideal.