ElecAS

UPS & Battery Sizing Calculator

Size a UPS battery bank from critical load (kW), backup time and DC bus voltage — returns required Ah, blocks per string, parallel strings, total capacity, achieved autonomy and indicative weight / footprint.

Why this page matters

Size a UPS battery bank from critical load (kW), backup time and DC bus voltage — returns required Ah, blocks per string, parallel strings, total capacity, achieved autonomy and indicative weight / footprint. This static content is published so the canonical route has meaningful crawlable HTML even before the interactive application hydrates.

Who this page is for

Electrical engineers, critical-power designers, data-centre engineers, contractors and specifiers sizing UPS battery banks for Australian and New Zealand installations.

Relevant standards

  • IEEE 485
  • IEEE 1184
  • IEEE 1189
  • AS 62040
  • AS/NZS 3000:2018

What this tool helps with

  • IEEE 485 / 1184 / 1189 constant-current sizing with aging factor (1.25 default for 80% end-of-life capacity), temperature derating and engineering design margin.
  • Supports VRLA (sealed lead-acid), LiFePO4 (lithium iron phosphate), Li-Ion (NMC / NCA) and Ni-Cd (vented) chemistries with preset nominal and end-of-discharge voltages.
  • Phase-aware DC bus voltage selector (48 / 96 / 120 / 192 / 240 V single-phase, 240 / 384 / 480 V three-phase) with plain-English guidance for each option.
  • Solves blocks in series per string, parallel strings, total installed Ah / kWh and the autonomy the bank actually delivers at the design load.
  • Indicative bank weight, volume and floor footprint per chemistry for early layout and plant-room planning.
  • Export a branded PDF report with project details, hero result card, system parameters, sizing breakdown, battery bank configuration, warnings and method & standards references.

Reviewed by

Wisam Tozah — Associate Electrical Engineer. B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus). LinkedIn.

Frequently asked questions

How do I size a UPS battery bank?
Start with the critical load (kW), required backup time (minutes), UPS inverter efficiency and DC bus voltage. Convert to DC load current (P_dc = kW ÷ efficiency, I_dc = P_dc ÷ V_dc), multiply by backup hours to get raw Ah, then apply aging factor (typ. 1.25), temperature derating and design margin. Blocks per string = ⌈V_dc ÷ V_unit⌉ and parallel strings = ⌈Required Ah ÷ Block Ah⌉ — this is the IEEE 485 / 1184 constant-current approximation the calculator uses.
What is the IEEE 485 aging factor and why is it 1.25?
IEEE 485 requires oversizing the battery so it still meets the load at end-of-life, commonly defined as 80% of rated capacity (1 ÷ 0.8 = 1.25). This aging factor is applied on top of the raw Ah demand so that a battery at end-of-life still delivers the specified backup time. Lithium banks often use lower factors (1.1 – 1.2) with tighter BMS monitoring.
Which battery chemistry is best for a UPS — VRLA, LiFePO4, Li-Ion or Ni-Cd?
VRLA is cheapest with 5 – 10 year life, best for short backups up to 30 minutes. LiFePO4 (lithium iron phosphate) is the most popular choice today for 10 – 15 year life, compact footprint and safe chemistry. Li-Ion (NMC / NCA) has the highest energy density but needs a certified BMS and fire-safety controls. Ni-Cd (vented) is heavy-duty, tolerates extreme temperatures and lasts 20+ years — typical for rail, switchyard and substation DC.
What DC bus voltage should I pick for my UPS?
48 V / 96 V / 120 V suit small telecom and single-phase UPS up to ~3 kVA. 192 V / 240 V are common for 5 – 20 kVA single-phase UPS. 240 V also covers entry-level 3-phase UPS ≤ 20 kVA. 384 V is the most common 3-phase UPS DC bus for 20 – 200 kVA, and 480 V is used for large data-centre and industrial 3-phase UPS. The calculator filters the DC bus options by the phase you select.
Why does the calculator need UPS efficiency and how does it affect battery size?
The battery has to supply the inverter input, not the AC output. If the inverter is 94% efficient, the DC side draws 1 ÷ 0.94 ≈ 6.4% more kW than the AC load. Higher efficiency gives a smaller battery for the same autonomy. Typical online (double-conversion) UPS efficiency is 92 – 96%; confirm with the manufacturer data sheet for the selected duty point.
How do aging, temperature and design margin combine in the sizing?
The calculator multiplies the raw Ah by aging × (1 + design margin %) and divides by the temperature derating factor. Aging 1.25 covers capacity fade to 80%, temperature derating < 1.0 accounts for operation below 25 °C (battery capacity falls with cold), and design margin adds engineering headroom (typ. 10 – 20%) for unmeasured losses, future load creep and commissioning tolerance.
Does the calculator work for single-phase (230 V) and three-phase (415 V) UPS?
Yes. Select 1-Phase or 3-Phase; the AC load current is computed with the correct phase factor (I = kVA × 1000 ÷ V for single-phase, I = kVA × 1000 ÷ (√3 × V) for three-phase) per AS 60038 nominal voltages. The DC bus options are filtered to show only voltages typical for the selected phase.
What warnings does the calculator check?
The calculator flags an end-of-discharge string voltage that is too close to the DC bus (inverter under-voltage risk), UPS efficiency below 85% (verify manufacturer data), VRLA banks sized beyond 60 minutes (consider lithium for long autonomy) and block / bus voltage mismatches where cells in series do not divide evenly into the DC bus — with a specific fix suggesting either changing the DC bus or the Unit Nominal voltage.
Which Australian and international standards apply to UPS battery installations?
IEEE 485 (vented lead-acid) and IEEE 1189 (VRLA) are the reference sizing methods; IEEE 1184 is the UPS-specific sizing guide. AS 62040 covers UPS safety, EMC and performance. AS/NZS 3000:2018 (Wiring Rules) applies to the AC installation, including battery-room ventilation and segregation. AS/NZS 4777.1 and AS/NZS 5139 may also apply for renewable-integrated UPS and lithium battery installations.
Can the calculator export a PDF report?
Yes. The Export PDF button generates a branded report including project details, the recommended battery bank (total Ah, cells per string, parallel strings, chemistry), system parameters, sizing breakdown with all factors, battery bank configuration, indicative weight and footprint, design-notes warnings, and a method / standards reference section.

Related ElecAS pages