---
title: "UPS & Battery Sizing Calculator: IEEE 485 Battery Bank and Autonomy"
url: "https://elecas.com.au/calculator/ups-battery"
description: "Free UPS battery sizing calculator to IEEE 485 & 1184: size VRLA, LiFePO4, Li-Ion and Ni-Cd battery banks from critical kW, backup time and DC bus voltage."
updated: "2026-09-16"
standards:
  - "IEEE 485"
  - "IEEE 1184"
  - "IEEE 1189"
  - "IEEE 1115"
  - "AS 62040"
  - "AS/NZS 5139:2019"
  - "AS/NZS 3000:2018"
author: "Wisam Tozah (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus))"
site: "ElecAS"
license: "https://elecas.com.au/terms"
usage: "Cite and link. Do not reproduce or train on this content."
---

# UPS & Battery Sizing Calculator: IEEE 485 Battery Bank and Autonomy

Source: https://elecas.com.au/calculator/ups-battery

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.

## Key facts

- IEEE 485 is the lead-acid sizing method, IEEE 1184 is the UPS battery guide, IEEE 1189 covers VRLA selection and IEEE 1115 covers nickel-cadmium sizing; lithium follows the manufacturer's discharge and C-rate data.
- The battery current is taken at the end-of-discharge string voltage, not the nominal bus: an inverter is a constant-power load, so a 240 V string ending at 210 V draws 14% more current at the end than at the start.
- Typical margins are 1.25 for aging (80% end-of-life) and 10% design margin, plus a temperature correction for a cold battery room.
- Nameplate Ah is a slow-rate figure (the 10-hour rate for lead-acid). Over 15 minutes a VRLA block delivers about half of it, so the calculator converts usable Ah to nameplate Ah with a Peukert exponent per chemistry before picking blocks.
- A 2N system has two independent sides, each with its own battery bank; N+1 shares one bank between the modules.
- The floor area quoted is a planned room, cabinet by cabinet, with the clearances AS/NZS 5139:2019 asks for: 600 mm in front of cabinets in a plant room, 900 mm working clearance in a battery room, 1200 mm between facing rows.

## 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
- IEEE 1115
- AS 62040
- AS/NZS 5139:2019
- AS/NZS 3000:2018

## What this tool does

- IEEE 485 / 1184 constant-current sizing with the battery current taken at the end-of-discharge string voltage, an aging factor (1.25 default for 80% end-of-life capacity), temperature derating and an engineering design margin.
- A Peukert discharge-rate correction per chemistry turns the usable Ah into the nameplate Ah to buy, so a 15-minute VRLA bank is not sized on its 10-hour rating.
- 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 (12 / 24 / 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.
- N, N+1 and 2N redundancy, with 2N sized as two independent banks; a planned rack, plant-room row or battery room with AS/NZS 5139 clearances, drawn to scale with its width and depth.
- Indicative bank weight and volume per chemistry for early layout and floor-loading conversations.
- Export a branded PDF report with project details, hero result card, system parameters, sizing breakdown, battery bank configuration, warnings and method & standards references.

## How to size a UPS and its battery bank

1. **Enter the critical load** — Type the load in kW, its power factor and the UPS efficiency, then pick single-phase (230 V) or three-phase (415 V). The calculator converts to kVA and AC current, adds the sizing margin (25% by default) and picks the next standard UPS frame.
2. **Set the backup time and the DC bus** — Enter the minutes of autonomy and choose the DC bus voltage from the list for your phase: 240 V is the common single-phase bus, 384 V the common three-phase one. The list is filtered to the voltages real UPS models use.
3. **Pick the chemistry and a block size** — Choose VRLA, LiFePO4, Li-Ion or vented Ni-Cd. The unit voltage, end-of-discharge voltage and Peukert exponent follow from it. The block dropdown marks the smallest standard block that meets the nameplate Ah in one string, and shows the parallel count for every other size; pick Custom to enter a datasheet value.
4. **Check the advanced factors if the room is not standard** — Open the advanced settings to change the aging factor (1.25), the temperature factor (1.0 at 25 °C), the design margin (10%) or the Peukert exponent. Set the exponent to 1.0 to size on nameplate Ah alone.
5. **Read the bank, the drawing and the room** — The result card shows nameplate Ah per string, usable Ah, blocks in series and strings in parallel, installed Ah and kWh, the achieved autonomy and the battery current at end of discharge. Below it the single-line diagram and the footprint sketch update with every change; the room's width and depth are on the sketch. Export the PDF for the full working.

## UPS battery sizing: IEEE 485, IEEE 1184 and the discharge rate in practice

### What the calculator actually does

The ElecAS UPS battery sizing calculator takes one critical load in kW at a power factor, one backup time in minutes, the UPS efficiency, a DC bus voltage and a battery chemistry, and returns the UPS frame and the battery bank. It is a single-step, constant-current sizing: there is no stepped discharge profile and no per-section iteration. The frame is the next standard rating above the load kVA plus a sizing margin, on a ladder from 1 kVA to 1200 kVA; above that the calculator says how many 1200 kVA modules to parallel.

The battery side runs in a fixed order. The DC power is the load divided by the inverter efficiency. The string is a whole number of units reaching the bus, and its end-of-discharge voltage is that count times the end voltage per unit. The design current is the DC power divided by that end-of-discharge voltage, because a constant-power inverter draws its highest current when the string is lowest. That current times the backup time, times the aging factor and the design margin, divided by the temperature factor, is the usable ampere-hours the string must deliver.

### From usable Ah to the nameplate on the block

A block's nameplate capacity is quoted at a slow rate: the 10-hour rate for lead-acid, the 5-hour rate for nickel-cadmium, the 1-hour rate for most lithium modules. Discharged over 15 minutes, a 100 Ah VRLA block gives nowhere near 100 Ah; roughly half, because the faster the discharge the less of the active material takes part. Sizing against nameplate Ah at a UPS rate under-sizes the bank by about that factor, and the tool used to do exactly that.

The calculator applies Peukert's relation, I^k × t = constant, to convert. The available fraction of nameplate is (t / T_ref)^(1 − 1/k), capped at one, with k defaulting to 1.2 for VRLA, 1.05 for LiFePO4 and Li-Ion and 1.1 for vented Ni-Cd, all editable in the advanced settings and switched off at k = 1. The nameplate ampere-hours to buy per string is the usable figure divided by that fraction, and the block size and parallel-string count are chosen against it. The report shows both figures so the arithmetic can be checked against the manufacturer's discharge table, which remains the authority for a critical installation.

### Lithium chemistries

LiFePO4 and Li-Ion hold their nameplate at high rates far better than lead-acid, which is why the same method with k near 1.05 gives close to the catalogue figure. What governs a lithium string in UPS duty is usually the continuous discharge rating instead: a 15-minute autonomy puts a string at 2C to 4C, and general-purpose LiFePO4 modules are often limited to 1C. The calculator reports the C-rate of each string and notes when it passes 1C so the module's datasheet is checked, but it does not enforce a limit, because UPS-grade lithium is built for these rates.

Depth-of-discharge limits for lithium are set by the battery management system, not by the sizing method; the end-of-discharge voltage per cell is the input that stands in for it here.

### Why the installed bank is always larger than the calculation

The ampere-hour figure the method produces is a requirement, not a purchasable quantity. Batteries come in discrete block capacities, and a string must contain a whole number of blocks reaching the DC bus voltage, so the installed bank rounds up twice: once to the next block size and again to the next whole string. A requirement that lands just above a boundary can cost a full extra string.

Three further allowances sit on top of that, and they compound rather than overlap. A design margin covers load growth over the installation's life. An aging factor (conventionally 1.25 for lead-acid) covers the fact that a battery at end of useful life delivers about 80% of its rated capacity and must still meet the autonomy on its last day, not its first. A temperature correction applies where the battery room runs below the 25 °C the rating assumes, since capacity falls with temperature.

This is why the achieved autonomy reported after rounding is the number worth quoting rather than the requirement that produced it: it is what the bank will actually deliver, with the same factors run backwards.

### Redundancy, and the room

N is one module carrying the load. N+1 is two modules, each rated for the full load, on a common battery bank, so the bank is sized once and the UPS line-up doubles. 2N is two independent systems with no shared parts, so each side gets its own bank: blocks, weight and floor area double, and the calculator says so.

The floor area is a planned layout, not a rule of thumb. A small system is one 19-inch rack with 600 mm in front of it. A mid-size system is a row of UPS and battery cabinets in a plant room with a 600 mm access strip, per AS/NZS 5139:2019 Section 5. A large one is a dedicated battery room: the UPS row faces the first battery row across a 1200 mm aisle, further rows stand back to back in pairs between aisles, and an even last row facing the wall gets its own 900 mm, per Section 6. The sketch draws that room, and the width times depth it prints is the area in the results and the report.

The bank determines how long the load rides through, and nothing else. The DC cabling between the bank and the UPS has to be sized for the end-of-discharge current, which at a low bus voltage and a high load is substantial, and on voltage drop, because the UPS has a minimum acceptable input. Ventilation for hydrogen, spill containment for flooded cells and thermal-runaway separation for lithium come from AS/NZS 5139 and the building code, not from the sizing method.

## Key terms

- **Autonomy (backup time)** — How long the UPS must support the critical load on battery alone. It drives the ampere-hour requirement roughly proportionally, so doubling the runtime roughly doubles the bank.
- **DC bus voltage** — The nominal battery voltage the UPS expects. It sets how many blocks go in series per string, and therefore the shape of the bank for a given capacity.
- **End-of-discharge voltage** — The per-unit voltage at which the UPS shuts down on low battery, typically 1.75 V per lead-acid cell (10.5 V per 12 V block) or 2.8 V per LiFePO4 cell. The string voltage at that point is what the battery current is calculated at.
- **String** — A set of battery blocks in series reaching the DC bus voltage. Strings are then paralleled to reach the required capacity.
- **Usable Ah** — The ampere-hours the string has to deliver over the backup time at the design current, after the aging, temperature and design-margin factors.
- **Nameplate Ah** — The capacity printed on the block, quoted at a slow reference rate. The calculator divides usable Ah by the discharge-rate factor to get the nameplate Ah to buy, and picks blocks against that.
- **Peukert exponent** — The constant k in I^k × t = constant that says how much capacity a battery loses when discharged fast. About 1.2 for VRLA, 1.05 for lithium, 1.1 for vented Ni-Cd; 1.0 switches the correction off.
- **C-rate** — Discharge current as a multiple of nameplate capacity: a 100 Ah string discharging at 250 A runs at 2.5C. Every block has a continuous C-rate limit on its datasheet.
- **IEEE 485** — The recommended practice for sizing lead-acid batteries for stationary applications: the source of the aging, temperature and design-margin factors this calculator applies.
- **IEEE 1184** — The guide for batteries in UPS systems, including the constant-current approximation of a constant-power inverter load taken at the end-of-discharge voltage.

## 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 power (P_dc = kW ÷ efficiency) and take the current at the end-of-discharge string voltage (I_dc = P_dc ÷ V_eod), where a constant-power inverter draws the most. Multiply by backup hours to get raw Ah, apply the aging factor (typ. 1.25), temperature derating and design margin for the usable Ah, then divide by the discharge-rate factor for the nameplate Ah to buy. Units per string = ⌈V_dc ÷ V_unit⌉ and parallel strings = ⌈Nameplate 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). The tool defaults to the legacy 415 V still widely in service, though AS 60038 gives 230/400 V as the nominal. The DC bus options are filtered to show only voltages typical for the selected phase.

### What warnings does the calculator check?

Warnings, which need a change: an end-of-discharge string voltage within 5% of the DC bus (inverter under-voltage risk), UPS efficiency below 85% (verify manufacturer data), a string more than 2% off the DC bus (with the nearest bus voltages that would fit), a frame loaded past 90%, and a load above the largest single frame (with the number of modules to parallel). Notes, which are advice: how much nameplate the discharge rate costs, a string running above 1C, VRLA beyond 60 minutes (consider lithium), and the second bank a 2N system carries.

### Which Australian and international standards apply to UPS battery installations?

IEEE 485 (lead-acid sizing) and IEEE 1189 (VRLA selection) are the reference methods, IEEE 1184 is the UPS-specific battery guide and IEEE 1115 covers nickel-cadmium. AS 62040 covers UPS safety, EMC and performance. AS/NZS 3000:2018 (Wiring Rules) applies to the AC installation. AS/NZS 5139:2019 sets the clearances the footprint sketch draws: 600 mm access in front of cabinets in a plant room and 900 mm working clearance in a battery room. AS/NZS 4777.1 may also apply for renewable-integrated UPS.

### Can the calculator export a PDF report?

Yes. The Export PDF button generates a branded report including project details, the recommended UPS frame and battery bank (nameplate and usable Ah, units per string, parallel strings, banks, chemistry), the load conversion with the end-of-discharge current, the sizing factors including the discharge-rate factor, the bank configuration, indicative weight and the planned room with its dimensions, the single-line diagram, warnings and notes, and a method / standards reference section.

### Which IEEE standard applies to my UPS battery sizing?

IEEE 485 is the sizing method for lead-acid batteries, both vented and VRLA. IEEE 1184 is the UPS-specific battery guide, IEEE 1189 covers selecting VRLA batteries, and IEEE 1115 covers sizing nickel-cadmium. Lithium chemistries (LiFePO4, Li-Ion) follow manufacturer guidance on discharge and C-rate: the ElecAS calculator applies the same factors with a chemistry-specific rate correction.

### Why is the battery current higher than kW divided by the bus voltage?

Because the inverter is a constant-power load and the string voltage falls as it discharges. The calculator takes the current at the end-of-discharge string voltage, where it peaks: 53 kW on a 240 V string that ends at 210 V is 253 A, not 222 A. That current sizes the bank, and it is also what the DC cable and the battery fuse have to carry.

### Why does the calculator ask for more nameplate Ah than the usable Ah it worked out?

Nameplate Ah is quoted at a slow rate (10 hours for lead-acid). A UPS discharge is fast, and the faster the discharge the less of the nameplate is available: a 100 Ah VRLA block gives about 54 Ah over 15 minutes. The calculator converts with a Peukert exponent per chemistry and picks blocks against the nameplate figure, so the strings you buy deliver the usable Ah you need.

### What aging margin should I use for a 20-year battery?

A typical aging margin is 1.25 (80% end-of-life capacity). Some high-reliability installations use 1.43 (70% end-of-life) for substation-grade applications. The aging margin multiplies the required capacity, so a 100 Ah profile at 1.25 aging requires 125 Ah before the rate correction.

### How does temperature affect battery sizing?

Lead-acid capacity drops below 25 °C and rises above 25 °C, but cycle life drops sharply above 30 °C. Lithium cell capacity is more stable across temperature but charge / discharge current capability drops below 0 °C. You enter the temperature correction factor yourself. The calculator starts at 1.0 for 25 °C and divides the required capacity by whatever figure you set, so lower it for a cold battery room using the manufacturer temperature table for your chemistry.

### Does 2N double the batteries?

Yes. 2N is two independent UPS systems, each carrying the full load with its own battery bank, so the calculator doubles the blocks, the installed kWh, the weight and the battery floor area. N+1 adds a second UPS module on a common bank, so only the UPS line-up doubles.

### Can lithium replace lead-acid in an existing UPS without resizing?

Usually no. Lithium nameplate Ah is closer to usable Ah than lead-acid nameplate Ah, so a like-for-like replacement may give more runtime than required (wasteful) or trip the UPS charger if the maximum charge current is exceeded. Always resize per the lithium chemistry, and check the module's continuous C-rate against the string current the calculator reports.

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## About this page

Written and reviewed by Wisam Tozah, Associate Electrical Engineer (B.Eng (Electrical), MIEAust, CPEng, NER, NSW DBP, NSW PRE, APEC, IntPE(Aus)), Sydney, Australia. How the calculations are tested and reviewed: https://elecas.com.au/verification. Profile: https://elecas.com.au/creator.

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