How to Size a UPS (kVA + Battery)
UPS sizing has two independent halves that engineers often mix up: the power rating (kVA/kW) — can the UPS carry the load — and the battery bank — for how long. A 10 kVA UPS can have 5 minutes or 2 hours of runtime depending entirely on the batteries behind it. This calculator solves both, the way a consultant's sizing sheet does.
Step 1 — UPS kVA Rating
Load VA = Load W ÷ Power Factor
Design VA = Load VA × (1 + Growth %)
Required kVA = Design VA ÷ Max Loading % → next standard rating
Two margins matter: growth (20–30% — loads always increase over the UPS's 10-year life) and loading (design at 75–80% of the rating, never 100% — headroom for inrush, load steps and reliability). Also check the UPS's kW rating: modern units are rated at 0.9–1.0 output PF, and the load must not exceed kVA or kW, whichever comes first.
Step 2 — Battery Bank
Battery Wh = Load W × (Runtime ÷ 60) ÷ ηinverter ÷ DoD × (1 + Aging %)
String Ah = Wh ÷ VDC · Blocks = VDC ÷ 12
The 25% aging margin follows IEEE battery-sizing practice — the bank must still deliver the runtime near end of life. Important: the energy method is preliminary; at short runtimes (5–15 min) batteries discharge at high rates where usable capacity drops (Peukert effect), so final selection must use the manufacturer's constant-power runtime charts.
Worked Example: 8 kW Server Room
- Load: 8 kW @ 0.9 PF = 8.9 kVA → +25% growth = 11.1 kVA
- At 80% max loading: 11.1 ÷ 0.8 = 13.9 kVA → select 15 kVA UPS (runs at ~74%)
- Battery for 15 min: 10 kW × 0.25 h ÷ 0.92 ÷ 0.80 × 1.25 ≈ 4.25 kWh
- At 192V DC: ≈ 22 Ah string = 16 × 12V blocks of ~26 Ah (next standard size)
Design Tips
- Generator + UPS: with standby generation, 5–15 minutes of autonomy is enough to ride through transfer; spending on longer runtime is wasted money.
- No generator: size for graceful shutdown (servers) or the longest credible outage (security/ELV systems often need 30–60+ minutes).
- Don't chain margins blindly: growth 25% + loading 80% already gives ~56% real headroom over today's load — adding more "just in case" produces an oversized, lightly-loaded UPS running at poor efficiency.
- Check input requirements too: the UPS input current (with battery recharge) sizes the upstream breaker and cable — see our Cable Size Calculator and Breaker Size Calculator.
- ELV head-ends: NVRs and access controllers often have their own 12/24V standby batteries — coordinate so you're not double-buying autonomy. For fire alarm panels, use the dedicated Fire Alarm Battery Calculator (NFPA 72 method differs).
Frequently Asked Questions
How do you calculate UPS size?
Load VA (= W ÷ PF) × (1 + growth) ÷ max loading → next standard kVA. Example: 8 kW @ 0.9 PF, +25%, @80% → 13.9 → 15 kVA.
kVA vs kW — what's the difference?
kVA is apparent power, kW = kVA × PF is real power. A UPS has both ratings; your load must fit within both.
How is battery backup time calculated?
Wh = W × hours ÷ inverter efficiency ÷ DoD × aging factor; Ah = Wh ÷ DC voltage. Verify short runtimes against vendor constant-power charts.
Why load a UPS to only 80%?
Headroom for inrush and load growth, longer battery runtime, less inverter stress — 75–80% is standard design practice.
What runtime should I choose?
5–15 min with a generator (transfer ride-through); 30–60+ min without one, or enough for graceful server shutdown.
Disclaimer: Preliminary sizing only. Final UPS and battery selection must be verified against manufacturer kW/kVA ratings, constant-power runtime charts and site conditions.