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UPS and Battery Backup Sizing

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The critical load needs backup, but oversizing burns capex and undersizing fails at the worst minute. Size kVA and battery Ah from the runtime you actually need.

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1Critical load and runtimeThese define the load the UPS must carry and for how long.
kWUse measured critical load, not the whole building load.
PFUse equipment PF or UPS design assumption.
minRuntime needed to ride through cuts or start alternate supply.
2Battery assumptionsThese decide how much Ah is needed from the battery bank.
VdcUse the DC bus or battery bank voltage from the UPS design.
%Usable battery depth. Lower values need larger batteries.
%Use the UPS or inverter efficiency at expected load.

The formula we use

kVA = kW/pf

Ah = (kW*1000 * minutes/60) / (Vdc * DoD/100 * eff/100)

  • Recommend next standard UPS size up from calculated kVA.
  • Battery ageing, temperature, discharge rate and autonomy margin are not modelled.
  • Critical equipment may need redundancy and bypass design.

Common questions

Why is UPS size in kVA higher than kW?

UPS kVA depends on load power factor. kVA equals kW divided by PF.

What is depth of discharge?

Depth of discharge is the usable share of battery capacity. Lower DoD preserves life but needs more Ah.

Does this include battery ageing?

No. Add design margin for ageing, temperature, discharge rate and criticality before purchase.

Related reading

UPS and battery charger lossesReadBattery storage BESS guideReadIT building and data room powerReadStandby losses after shutdownRead