Generator UPS Calculator
Size generator kVA for UPS-backed critical load and plan UPS bridge minutes until transfer completes.
Quick answer
Generator + UPS sizing pairs two problems: the generator must carry critical kW (plus optional battery recharge) after transfer, and the UPS must ride through crank and ATS delay. Example: 50 kW critical, 15 min bridge, PF 0.85, 5 kW recharge → about 93 kVA required → 100 kVA frame. Cross-check minutes in the UPS runtime calculator and kVA in the generator size calculator. Hub: UPS calculator.
Quick Generator UPS Calculator
Defaults: 50 kW critical, 15 min bridge, PF 0.85, 5 kW UPS recharge.
50 kW critical + 5 kW recharge · 15 min UPS bridge planning · PF 0.85 · 15% margin.
Advanced Generator UPS Calculator
Quick Examples
Generator + UPS Results
Engineering disclaimer
Planning estimate only. Generator start curves, harmonic loading, fuel storage, and parallel operation rules override spreadsheet kVA—confirm with OEM and site ATS settings.
Results
93 kVA required → 100 kVA frame — default example.
Explain this result (summary)
- Generator load: 55.0 kW total (50.0 kW critical + 5.0 kW UPS recharge).
- Generator kVA: 93.0 kVA required → 100 kVA catalog frame
- UPS bridge: Plan 15.0 min ride-through (gen start + ATS delay).
Operational guidance
Enter UPS runtime to verify bridge
Planning bridge is 15.0 min—cross-check in the UPS runtime calculator when battery data is known.
Bridge window vs typical UPS runtime
| Bridge target | Typical use |
|---|---|
| 5–10 min | Fast ATS, proven gen start |
| 10–15 min | Industrial / data center default |
| 15–30 min | Cold weather or uncertain start |
People also ask
- What generator size for UPS-backed load? Size the generator for critical kW (÷ PF) plus optional UPS recharge, surge factor, and margin—UPS covers the gap until the generator accepts load.
- How long must UPS run before the generator starts? Plan for generator crank + ATS transfer time—often 30–90 seconds minimum, many sites target 10–15 minutes total.
- Is this the same as the generator size calculator? Generator size sizes any load. This tool adds UPS bridge minutes and recharge load for generator + UPS workflows.
Generator + UPS planning guidance
- Bridge time: UPS must cover generator crank + ATS transfer—often longer than nameplate start time alone.
- Recharge: Add UPS rectifier load if batteries recharge on generator—omit if recharge is deferred.
- kVA vs kW: Divide total kW by PF, then apply surge and margin—same basis as generator size.
- Runtime check: Enter minutes from UPS runtime to confirm bridge coverage.
Typical scenarios
- Data center: 200 kW critical, 15–20 min bridge, recharge on gen—validate with facility ATS sequence.
- Industrial line: 50–100 kW, 10 min bridge, motor inrush on transfer—use higher surge factor.
- Telecom / edge: Short bridge if gen is always warm—still confirm cold-start worst case.
Generator UPS formula
Required kVA = (Critical kW + Recharge kW) ÷ PF × Surge × (1 + Margin%)
Planning bridge (min) = Bridge override OR (Gen start sec + Transfer sec) ÷ 60
Frequently Asked Questions
What generator size for UPS-backed load?
Size for critical kW ÷ PF plus optional recharge, surge, and margin. UPS covers the gap until the generator accepts load.
How long must UPS run before the generator starts?
Plan for crank plus ATS delay—often 30–90 seconds minimum; many sites target 10–15 minutes total.
Is this the same as the generator size calculator?
Generator size sizes any load. This tool adds UPS bridge timing and optional recharge for integrated workflows.
Should I include UPS recharge on the generator?
Many designs add 5–20% of critical kW for battery recharge after transfer. Use 0 if recharge is deferred.
What is the next step?
Verify minutes in the UPS runtime calculator or cross-check kVA in the generator size calculator.
How it works
Generator plus UPS sizing combines generator kVA requirements with UPS bridge minutes—the interval where batteries carry load until the generator is online and the transfer switch completes.
Critical kW converts to kVA using power factor, then optional UPS recharge, surge factor, and planning margin scale the generator frame. Bridge minutes come from your planning target or from generator start plus ATS transfer delay.
Enter UPS runtime from the runtime calculator to verify the battery plant covers the planned bridge window before procurement.
Formula and sources
Required kVA = (Critical kW + Recharge kW) ÷ PF × Surge × (1 + Margin%); Bridge min = override OR (Gen start + Transfer) ÷ 60
Catalog generator frames use standard commercial kVA sizes.
OEM start curves and harmonic loading may require larger frames than spreadsheet kVA.
Worked examples
- 50 kW with 15 min bridge
55 kW total with 5 kW recharge at PF 0.85, 1.25 surge, 15% margin yields about 93 kVA required and a 100 kVA catalog frame.