Sizes the required UPS kVA rating (and an approximate battery requirement) from a critical load, design margin, and redundancy configuration. Shares its N-unit redundancy model with the Transformer Sizer; for detailed battery design, see the dedicated Battery & DC System Sizing calculator's full IEEE 485 method.
Standards followed
IEEE 485 (referenced battery methodology)General N-unit redundancy convention ("N+1")
What it can do
- Sizes from kW+PF or direct kVA, with a design margin.
- N-1/"N+1" redundant frame sizing — any (N−1) of N identical UPS frames carries the full load.
- Reports real-power (kW) capability at the UPS's rated output power factor.
- Approximate battery energy and Ah estimate for the stated backup time, DoD and bus voltage.
Example problem
Size an N=2 redundant UPS system for a 200kW, 0.9 PF critical load with 20% margin, 15-minute backup at 0.8 DoD, 92% inverter efficiency and a 480V DC bus.
- Design load = 200 / 0.9 = 222.2 kVA.
- Margined load = 222.2 × 1.20 = 266.7 kVA.
- With N=2, any 1 of 2 frames must carry the full load alone → per-frame requirement = 266.7 kVA.
- Round up to the nearest standard UPS size → 300 kVA per frame (270kW capability at 0.9 rated PF).
- Usable battery energy = 200kW × 0.25h = 50 kWh.
- Nameplate battery energy = 50 / (0.8 × 0.92) ≈ 67.935 kWh.
- Approximate capacity at 480V DC ≈ 67,935Wh / 480V ≈ 141.5 Ah.
300 kVA UPS, ≈68 kWh nameplate battery, ≈141.5 Ah.
Want the full step-by-step version?
This battery figure is a quick sizing estimate only \u2014 for a full IEEE 485 duty-cycle section-by-section battery design, use the dedicated Battery & DC System Sizing calculator.