UPS size and battery run time
Size a UPS and its battery for a run time.
Reads AS/NZS 3000:2018. Tables checked 2 Oct 2026.
Try a job 4
Each job fills in example values. Replace them with your values.
- Size a UPS for a comms rackThe UPS and battery to keep a comms rack and security system up for half an hour.
- Find the battery for an hour on UPSHow much battery an office rack needs to run for an hour.
- Size a UPS for a small data hallA three-phase UPS for forty racks for ten minutes, and its input current.
- Pick a UPS for a home officeThe UPS for a computer and router to ride a short outage.
Inputs
Worked example with the default inputs. The live calculator replaces it when the page loads.
- Loads on the UPS
- 3 rows
- Headroom for growth
- 25 %
- UPS output power factor
- 0.9
- Run time on battery
- 15 min
- Battery bus voltage
- 96 V
- Inverter efficiency on battery
- 92 %
- Ageing factor
- 1.25
- Recharge time
- 8 h
- Input phases
- Single phase
- Input voltage
- 230 V
- UPS efficiency on mains
- 94 %
- Input power factor
- 0.99
UPS rating, at least
1.86kVA
- Typical size
- 2 kVA
- Load
- 1,340W
- Battery energy
- 455Wh
- Battery capacity at the bus
- 4.7Ah
- Input current
- 6.5A
This result is a design aid. Verify it against the current standard.
Calculation steps
- Load, real power1,340W
- Load, apparent power1,424.6VA
- kVA for the load with 25 % headroom1.78kVA
- kVA for its kW at a UPS power factor of 0.91.86kVA
- UPS rating, at leastResult1.86kVA
- Typical size2 kVA
- Battery energy for 15 min, with ageing455Wh
- Battery capacity at 96 V4.7Ah
- Input power with recharge in 8 h1,488W
- Input current6.5A
About this calculator
How this is calculated
A UPS has two sizes that matter: the power its inverter can deliver, and the energy its battery holds. This tool works both from a list of what the UPS supplies, then the current it draws from the supply.
You list each load with how many there are, the real power each draws in watts and its power factor. The tool adds the watts, and it adds each load's apparent power. With your headroom for growth on top, the UPS needs at least that many kVA, and its kW rating, its kVA times its output power factor, must cover the watts. The larger of the two is the rating. The tool also names the next typical size up; the sizes are common ratings, not a standard's list.
The battery must hold the load's watts for the run time, divided by the inverter's efficiency, times an ageing factor so it still lasts when old. Divided by the bus voltage that gives Ah, but at the run time's rate: a battery gives far less than its 10 or 20 hour Ah over a short run, so the maker's discharge table decides the battery.
The input is the load through the UPS's efficiency plus the power to recharge the battery in the time you set. AmpSize does not hold AS 62040, so the headroom, efficiencies and ageing factor are your assumptions. The result names the AS/NZS 3000 rules for converters.
| Step | Table or clause | What it decides |
|---|---|---|
| Load | Watts added, and each load's apparent power | The power the UPS supplies |
| Rating | The larger of the kVA and the kW at the output power factor | The UPS rating with headroom |
| Battery energy | Load times run time, over efficiency, times ageing | The energy at the battery bus |
| Battery capacity | Energy over the bus voltage, at the run time's rate | The Ah to look up in a maker's table |
| Input current | Load over efficiency, plus recharge | The current for the supply cable |
| Installation | AS/NZS 3000 Cl 4.12.4.2, Cl 4.12.5.1, Cl 4.12.5.2.2 and Cl 4.12.7 | The switch, protection, RCD and neutral rules |
Worked examples
- An office rack: two servers, three switches and storage, 15 min needs 1.86 kVA, so a 2 kVA unit, with 4.7 Ah at 96 V.
- The same rack for an hour still needs 1.86 kVA and a 2 kVA unit, but the battery holds 1,821 Wh.
- A comms rack and security system, 30 min on a 192 V battery needs 4.17 kVA, a 5 kVA unit, with 10.6 Ah at the bus.
- A small data hall of forty 1.2 kW racks, 10 min, three-phase needs 61.86 kVA, an 80 kVA unit, drawing 76.6 A a phase.
- A home office PC and router, 10 min needs 0.51 kVA, so a 1 kVA unit.
Limits
The battery figure is the energy and Ah at the run time's rate. It does not pick cells: read the maker's constant-power discharge table at your end voltage and temperature. The tool does not model a UPS in parallel, redundancy such as N+1, the bypass supply, or a generator behind the UPS; for that, size the generator for the UPS's input.
See also: Generator sizing, Battery and Cable size.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
How do I size a UPS?
Add the watts of everything it supplies, and their VA. Add headroom for growth. The UPS kVA must cover the VA, and its kW rating, kVA times its output power factor, must cover the watts.
How much battery do I need for a run time?
The energy is the load in watts times the hours, divided by the inverter efficiency, times an ageing factor. Divide by the bus voltage for Ah, at the run time rate, then read the maker discharge table.
Why is the Ah so different from the battery label?
A label gives the Ah over 10 or 20 hours. Over 15 minutes the same battery gives much less, so a UPS battery is chosen from a constant-power discharge table, not the label.
What size cable feeds a UPS?
Size it for the input current: the load through the UPS efficiency, plus the power to recharge the battery. The tool gives that current to carry into the cable size tool.
What does AS/NZS 3000 say about a UPS?
Cl 4.12 covers converters, including UPS. The neutral to the load stays unbroken through bypass, the output protection sits within 15 m, and a switch beside the batteries breaks their supply.
Clauses and tables this tool reads
- AS/NZS 3000:2018 Cl 4.12.4.2Switch for converter batteries
- AS/NZS 3000:2018 Cl 4.12.5.1Converter overcurrent protection
- AS/NZS 3000:2018 Cl 4.12.5.2.2RCDs on a converter output
- AS/NZS 3000:2018 Cl 4.12.7Neutral continuity through a UPS
- AS/NZS 3000:2018 Cl 4.12Electricity converters, including UPS
Every clause and table, with how each was verified
Tables last checked on 2 Oct 2026.
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