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?

The aging factor is 1.25 because IEEE 485 recommends sizing a battery to still carry the load when it has faded to 80% of its rated capacity, the point at which it is normally replaced (1 ÷ 0.8 = 1.25). It multiplies the raw ampere-hours, so a bank that needs 100 Ah new is sized for 125 Ah before the margin, temperature and discharge-rate corrections. For lithium, use the end-of-life capacity the manufacturer warrants: an 80% warranty figure is 1.25 again. The calculator starts at 1.25 for every chemistry and accepts any figure of 1.0 or more.

Which battery chemistry is best for a UPS: VRLA, LiFePO4, Li-Ion or Ni-Cd?

It depends on the life, the space and the budget. VRLA is the cheapest to buy, with a 5 to 10 year life, and suits short backups up to about 30 minutes. LiFePO4 (lithium iron phosphate) lasts 10 to 15 years in a fraction of the weight and space and is a common choice for a new UPS. Li-Ion (NMC / NCA) has the highest energy density but needs a certified BMS and fire-safety controls. Vented Ni-Cd is heavy, tolerates extreme temperatures and lasts 20 years or more, which is why rail, switchyard and substation DC use it. The calculator starts on LiFePO4 and loads each chemistry's unit voltage, end-of-discharge voltage and discharge-rate correction when you switch.

What DC bus voltage should I pick for my UPS?

Pick the DC bus the UPS manufacturer specifies for the frame. As a guide, 12 V and 24 V suit very small UPS and telecom shelves; 48 V, 96 V and 120 V small single-phase UPS up to about 3 kVA; 192 V and 240 V single-phase UPS from 5 to 20 kVA; 240 V also entry-level three-phase UPS up to 20 kVA; 384 V is the most common three-phase bus from 20 to 200 kVA; and 480 V is used for large data-centre and industrial three-phase UPS. The calculator lists only the buses typical of the phase you select, starting on 240 V single-phase or 384 V three-phase.

Why does the calculator need UPS efficiency and how does it affect battery size?

The battery feeds the inverter's DC input, not the AC output, so the inverter's losses come out of the battery too: at 94% efficiency the DC side draws 1 ÷ 0.94, about 6.4% more than the AC load. Higher efficiency gives a smaller battery for the same autonomy. Online (double-conversion) UPS efficiency is typically 92% to 96%; the calculator starts at 0.94 and warns below 0.85. Confirm the figure on the manufacturer's data sheet at the duty point.

Does the calculator work for single-phase (230 V) and three-phase (400 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 uses the AS 60038 nominal voltages, 230 V and 400 V. 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%, a load above the largest single frame (with the number of modules to parallel), and a frame picked by hand that is smaller than the load or its margin. Where one field clears a warning, the warning carries a button that sets it. 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 standards apply to a UPS battery installation?

The UPS is covered by the AS 62040 series and its batteries by the AS 3011 series: AS/NZS 3000:2018 Clause 7.8.2.15 sends secondary batteries for critical power continuity, UPS included, to AS 3011 because they sit outside AS/NZS 5139. AS/NZS 5139:2019 itself excludes a UPS to AS 62040 from its scope (AS/NZS 5139 Cl 1.1.1), so the planned battery room borrows its 900 mm working-side clearance (AS/NZS 5139 Cl 6.2.6.2) as a sound layout figure rather than a requirement, and the plant-room line-up keeps the 1000 mm service clearance UPS manufacturers ask for. AS/NZS 3000 governs the AC installation, and the sizing follows IEEE 485, 1184, 1189 and 1115.

Can the calculator export a PDF report?

Yes. Export in the toolbar downloads a branded PDF report once you have a free account: a calculation sheet with the installed battery and the autonomy verdict at the top, the inputs once, then every step of the working on a ruled ledger, from the load current and the frame through the input and fault currents, the end-of-discharge current, the IEEE 485 factors and the discharge-rate factor to the bank and the autonomy, each with its formula and figures. The single-line diagram and the planned room follow as vector drawings, then the warnings, the notes and the method. Download Project File saves the job itself, with no account needed.

How do I calculate UPS battery backup time (runtime)?

Backup time is the usable ampere-hours divided by the battery current: hours = usable Ah ÷ DC current at end of discharge, where that current is the load kW ÷ inverter efficiency ÷ end-of-discharge string voltage. The calculator reads the autonomy back from the bank it actually picks, rounded up to whole blocks and strings, with the aging, margin, temperature and discharge-rate factors still applied, so the minutes it reports are what an aged battery delivers at the design load; a new bank runs longer. The figure is marked against your target backup time.

What end-of-discharge voltage should I use for a UPS battery?

Use the end voltage the battery manufacturer rates the discharge to, matched to the UPS's DC under-voltage cut-off. The calculator starts at 10.5 V per 12 V VRLA block (1.75 V per cell), 2.8 V per LiFePO4 cell, 3.0 V per Li-Ion cell and 1.0 V per Ni-Cd cell. A lower end voltage releases more of the block's capacity, which the manufacturer's discharge table shows, but the inverter draws constant power, so it also raises the current at the end of the discharge. The calculator sizes the bank at the end-of-discharge string voltage and warns when that voltage is within 5% of the DC bus, where the inverter's under-voltage limit may trip first.

How many batteries do I need for a UPS?

Blocks per string = DC bus voltage ÷ block voltage, rounded up, and strings in parallel = nameplate Ah needed ÷ block Ah, rounded up; the total is blocks per string × strings, doubled for 2N. A 384 V bus on 12 V VRLA blocks is 32 blocks per string, and a 500 Ah requirement on 100 Ah blocks is five strings, 160 blocks. The calculator works out both, warns when the string lands more than 2% off the bus, and offers the bus voltage that fits.

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 ScaleSet 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: a 50 kW load through a 94% efficient inverter draws 53.2 kW from the battery, which is 222 A at a 240 V nominal string but 253 A at the 210 V it ends at. 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.

How does temperature affect battery sizing?

A cold battery delivers less: lead-acid capacity falls below 25 °C, and lithium loses charge and discharge current capability below 0 °C. Lead-acid capacity rises above 25 °C but its life drops sharply above 30 °C, so conservative practice takes no credit for a warm room. The temperature factor is the capacity at the coldest room temperature divided by the capacity at 25 °C, read from the manufacturer's temperature table for your chemistry. The calculator starts at 1.0 and divides the required capacity by it, so a factor of 0.9 for a cold room asks for about 11% more. A figure above 1.0 is read as 1.0: following IEEE 485, a warm room is credited nothing.

Does a large UPS need a separate battery room?

Not because it is large. What decides it is the battery: vented lead-acid and vented Ni-Cd give off hydrogen on charge and normally go in a dedicated, ventilated battery room, under AS 3011, which AS/NZS 3000 Cl 7.8.2.15 sends UPS batteries to. Sealed VRLA and lithium usually sit in cabinets beside the UPS until the bank grows to the point where fire separation (mostly for lithium, set by the fire engineer, the building code or the insurer), cooling (batteries want about 20 to 25 °C) and floor loading make a room of its own the better answer. On Auto the calculator gives vented cells and banks above about 120 kWh of VRLA or 200 kWh of lithium their own room, and the Batteries in control lets you set it to what the project actually requires.

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. A lithium bank needs a UPS whose charger and battery-monitoring settings are approved for that chemistry and its BMS, and lithium gives far more of its nameplate Ah at a UPS discharge rate than lead-acid does, so a like-for-like Ah swap oversizes the bank. Resize for the lithium chemistry, and check the module's continuous C-rate against the string current the calculator reports.

ScaleSet

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

Size a UPS frame and its battery bank from critical load (kW), backup time and DC bus voltage: returns the frame with its input and fault current, required Ah, blocks per string, parallel strings, total capacity, achieved autonomy and indicative weight / footprint.

Overview

Sizing a UPS battery bank starts from three numbers: the critical load in kW, how long it has to ride through, and the DC bus voltage the UPS runs at. From those this calculator returns the standard UPS frame, the battery current at the end-of-discharge voltage, the ampere-hours the string must deliver, the nameplate ampere-hours to buy once the discharge rate is allowed for, how many blocks that is per string and how many strings in parallel, the autonomy actually achieved once the bank is rounded to real blocks, the input current and the fault current on battery for the frame, and a planned plant room or battery room with its width and depth.

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: 600 mm in front of a rack, 1000 mm of service clearance in front of a plant-room line-up (the metre UPS makers ask for, and the 1.0 m AS/NZS 3000 Cl 2.10.2.2.1 sets in front of the bypass switchboard), and in a battery room 900 mm on a row's working side with 1200 mm between facing rows. The 900 mm is the AS/NZS 5139 battery-room figure (AS/NZS 5139 Cl 6.2.6.2) borrowed as a layout choice: AS/NZS 5139 excludes a UPS to AS 62040 (AS/NZS 5139 Cl 1.1.1), and AS/NZS 3000 Cl 7.8.2.15 sends UPS batteries to AS 3011.

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 3011
AS/NZS 3000:2018

What this tool helps with

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; the batteries in the UPS room or in a room of their own, decided by the chemistry and the energy rather than the frame size and open to override, drawn to scale as an indicative plan: 1000 mm of service clearance in front of a line-up, 900 mm on a battery row's working side (the AS/NZS 5139 battery-room figure, borrowed as a layout choice because that standard excludes a UPS, AS/NZS 5139 Cl 1.1.1), and a second door wherever AS/NZS 3000 Cl 2.10.2.2.2 asks for two exits.
The supply side of the frame: the input current per module, at full rated output with the charger recharging, for the input cable and device, and the fault current the inverter delivers on battery with the bypass unavailable, for the downstream disconnection check.
Auto picks the next standard frame; pick a frame by hand to specify the unit being bought, and every figure, from the input current to the room, follows it.
Indicative bank weight and volume per chemistry for early layout and floor-loading conversations.
Export a calculation sheet: the installed battery and the verdict at the top, the inputs once, every step of the working on a ruled ledger with its formula and figures, and the single-line diagram and room plan as vector drawings.

How to size a UPS and its battery bank

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 (400 V). The calculator converts to kVA and AC current, adds the sizing margin (25% by default) and picks the next standard UPS frame.
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.
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.
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 or warmer, lower for a cold room), the design margin (10%) or the Peukert exponent. Set the exponent to 1.0 to size on nameplate Ah alone.
Pick the frame, or leave it on Auto: Auto picks the smallest standard frame over the load and its margin. To specify the unit you are buying, pick its frame from the list, which shows how loaded each size would be; the input current, the fault current on battery and the room then follow that frame, and a warning says if the load or its margin does not fit it.
Read the bank, the drawing and the room: The results show the UPS frame and the installed kWh, then the working line by line: the load in kVA and amps, the frame and its loading, the input current per module, the fault current on battery, the battery current at end of discharge, the energy and nameplate Ah, the bank, the achieved autonomy and the room. Below it the single-line diagram and the footprint sketch update with every change. Export the PDF for the calculation sheet.

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

What the calculator actually does

The ScaleSet 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.

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, two for 2N, with the battery modules at the bottom and 600 mm in front of it. A mid-size system is one line-up in a plant room, the bypass switchboard at its head (on the wall up to 100 kVA, standing in the line above that), then the UPS and its matching battery cabinets, with 1000 mm of service clearance in front: the metre UPS makers ask for, which also gives the bypass board the 1.0 m AS/NZS 3000 Cl 2.10.2.2.1 sets in front of a switchboard. Where the batteries go is decided by what is in them, not by the size of the UPS: a 500 kVA UPS with its battery cabinets in one plant room is an ordinary installation. On Auto the calculator gives the batteries a room of their own when they are vented cells, which give off hydrogen on charge and are ventilated under AS 3011, or when the bank passes the size where fire separation, cooling and floor loading usually call for one (about 120 kWh of VRLA or 200 kWh of lithium). The Batteries in control overrides it either way, for when the fire engineer, the insurer or the client has decided. Kept in the UPS room, a bank too big for the line-up goes into rows across a 1200 mm service aisle from it. In a room of their own, the first battery row backs onto the dividing wall, further rows stand back to back in pairs, two rows that face each other share a 1200 mm aisle, and a row left facing a wall gets 900 mm. Each room is checked against AS/NZS 3000 Cl 2.10.2.2.2: a room whose bypass board or battery breaker is rated 800 A or more, taking the standard rating over its current, gets a second door, on the opposite wall. That 900 mm is the working side AS/NZS 5139 Cl 6.2.6.2 sets for a battery system room, used here as a sound layout figure: AS/NZS 5139 excludes a UPS to AS 62040 from its scope (AS/NZS 5139 Cl 1.1.1), and AS/NZS 3000 Cl 7.8.2.15 sends UPS batteries to AS 3011. 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 3011, the battery and UPS manufacturers 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.

Methods reviewed by

Wisam Tozah (CPEng, NER, MIEAust) for ScaleSet Pty Ltd. The methods are tested against the standards' published tables and worked examples before every release. See how they are verified. ScaleSet is a design aid: your inputs and results are not reviewed or certified, and responsibility for the design stays with the engineer who issues it. LinkedIn. Updated 4 November 2026.

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?: The aging factor is 1.25 because IEEE 485 recommends sizing a battery to still carry the load when it has faded to 80% of its rated capacity, the point at which it is normally replaced (1 ÷ 0.8 = 1.25). It multiplies the raw ampere-hours, so a bank that needs 100 Ah new is sized for 125 Ah before the margin, temperature and discharge-rate corrections. For lithium, use the end-of-life capacity the manufacturer warrants: an 80% warranty figure is 1.25 again. The calculator starts at 1.25 for every chemistry and accepts any figure of 1.0 or more.
Which battery chemistry is best for a UPS: VRLA, LiFePO4, Li-Ion or Ni-Cd?: It depends on the life, the space and the budget. VRLA is the cheapest to buy, with a 5 to 10 year life, and suits short backups up to about 30 minutes. LiFePO4 (lithium iron phosphate) lasts 10 to 15 years in a fraction of the weight and space and is a common choice for a new UPS. Li-Ion (NMC / NCA) has the highest energy density but needs a certified BMS and fire-safety controls. Vented Ni-Cd is heavy, tolerates extreme temperatures and lasts 20 years or more, which is why rail, switchyard and substation DC use it. The calculator starts on LiFePO4 and loads each chemistry's unit voltage, end-of-discharge voltage and discharge-rate correction when you switch.
What DC bus voltage should I pick for my UPS?: Pick the DC bus the UPS manufacturer specifies for the frame. As a guide, 12 V and 24 V suit very small UPS and telecom shelves; 48 V, 96 V and 120 V small single-phase UPS up to about 3 kVA; 192 V and 240 V single-phase UPS from 5 to 20 kVA; 240 V also entry-level three-phase UPS up to 20 kVA; 384 V is the most common three-phase bus from 20 to 200 kVA; and 480 V is used for large data-centre and industrial three-phase UPS. The calculator lists only the buses typical of the phase you select, starting on 240 V single-phase or 384 V three-phase.
Why does the calculator need UPS efficiency and how does it affect battery size?: The battery feeds the inverter's DC input, not the AC output, so the inverter's losses come out of the battery too: at 94% efficiency the DC side draws 1 ÷ 0.94, about 6.4% more than the AC load. Higher efficiency gives a smaller battery for the same autonomy. Online (double-conversion) UPS efficiency is typically 92% to 96%; the calculator starts at 0.94 and warns below 0.85. Confirm the figure on the manufacturer's data sheet at the duty point.
Does the calculator work for single-phase (230 V) and three-phase (400 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 uses the AS 60038 nominal voltages, 230 V and 400 V. 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%, a load above the largest single frame (with the number of modules to parallel), and a frame picked by hand that is smaller than the load or its margin. Where one field clears a warning, the warning carries a button that sets it. 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 standards apply to a UPS battery installation?: The UPS is covered by the AS 62040 series and its batteries by the AS 3011 series: AS/NZS 3000:2018 Clause 7.8.2.15 sends secondary batteries for critical power continuity, UPS included, to AS 3011 because they sit outside AS/NZS 5139. AS/NZS 5139:2019 itself excludes a UPS to AS 62040 from its scope (AS/NZS 5139 Cl 1.1.1), so the planned battery room borrows its 900 mm working-side clearance (AS/NZS 5139 Cl 6.2.6.2) as a sound layout figure rather than a requirement, and the plant-room line-up keeps the 1000 mm service clearance UPS manufacturers ask for. AS/NZS 3000 governs the AC installation, and the sizing follows IEEE 485, 1184, 1189 and 1115.
Can the calculator export a PDF report?: Yes. Export in the toolbar downloads a branded PDF report once you have a free account: a calculation sheet with the installed battery and the autonomy verdict at the top, the inputs once, then every step of the working on a ruled ledger, from the load current and the frame through the input and fault currents, the end-of-discharge current, the IEEE 485 factors and the discharge-rate factor to the bank and the autonomy, each with its formula and figures. The single-line diagram and the planned room follow as vector drawings, then the warnings, the notes and the method. Download Project File saves the job itself, with no account needed.
How do I calculate UPS battery backup time (runtime)?: Backup time is the usable ampere-hours divided by the battery current: hours = usable Ah ÷ DC current at end of discharge, where that current is the load kW ÷ inverter efficiency ÷ end-of-discharge string voltage. The calculator reads the autonomy back from the bank it actually picks, rounded up to whole blocks and strings, with the aging, margin, temperature and discharge-rate factors still applied, so the minutes it reports are what an aged battery delivers at the design load; a new bank runs longer. The figure is marked against your target backup time.
What end-of-discharge voltage should I use for a UPS battery?: Use the end voltage the battery manufacturer rates the discharge to, matched to the UPS's DC under-voltage cut-off. The calculator starts at 10.5 V per 12 V VRLA block (1.75 V per cell), 2.8 V per LiFePO4 cell, 3.0 V per Li-Ion cell and 1.0 V per Ni-Cd cell. A lower end voltage releases more of the block's capacity, which the manufacturer's discharge table shows, but the inverter draws constant power, so it also raises the current at the end of the discharge. The calculator sizes the bank at the end-of-discharge string voltage and warns when that voltage is within 5% of the DC bus, where the inverter's under-voltage limit may trip first.
How many batteries do I need for a UPS?: Blocks per string = DC bus voltage ÷ block voltage, rounded up, and strings in parallel = nameplate Ah needed ÷ block Ah, rounded up; the total is blocks per string × strings, doubled for 2N. A 384 V bus on 12 V VRLA blocks is 32 blocks per string, and a 500 Ah requirement on 100 Ah blocks is five strings, 160 blocks. The calculator works out both, warns when the string lands more than 2% off the bus, and offers the bus voltage that fits.
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 ScaleSet 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: a 50 kW load through a 94% efficient inverter draws 53.2 kW from the battery, which is 222 A at a 240 V nominal string but 253 A at the 210 V it ends at. 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.
How does temperature affect battery sizing?: A cold battery delivers less: lead-acid capacity falls below 25 °C, and lithium loses charge and discharge current capability below 0 °C. Lead-acid capacity rises above 25 °C but its life drops sharply above 30 °C, so conservative practice takes no credit for a warm room. The temperature factor is the capacity at the coldest room temperature divided by the capacity at 25 °C, read from the manufacturer's temperature table for your chemistry. The calculator starts at 1.0 and divides the required capacity by it, so a factor of 0.9 for a cold room asks for about 11% more. A figure above 1.0 is read as 1.0: following IEEE 485, a warm room is credited nothing.
Does a large UPS need a separate battery room?: Not because it is large. What decides it is the battery: vented lead-acid and vented Ni-Cd give off hydrogen on charge and normally go in a dedicated, ventilated battery room, under AS 3011, which AS/NZS 3000 Cl 7.8.2.15 sends UPS batteries to. Sealed VRLA and lithium usually sit in cabinets beside the UPS until the bank grows to the point where fire separation (mostly for lithium, set by the fire engineer, the building code or the insurer), cooling (batteries want about 20 to 25 °C) and floor loading make a room of its own the better answer. On Auto the calculator gives vented cells and banks above about 120 kWh of VRLA or 200 kWh of lithium their own room, and the Batteries in control lets you set it to what the project actually requires.
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. A lithium bank needs a UPS whose charger and battery-monitoring settings are approved for that chemistry and its BMS, and lithium gives far more of its nameplate Ah at a UPS discharge rate than lead-acid does, so a like-for-like Ah swap oversizes the bank. Resize for the lithium chemistry, and check the module's continuous C-rate against the string current the calculator reports.