How Many Batteries Does an Online UPS Need? (And How Long It Runs)
The battery count is fixed by the UPS's DC voltage; the Ah sets the runtime. The table for 1kVA–10kVA.
Quick answer
The number of batteries is fixed by the UPS's DC system voltage, not your choice: divide the DC voltage by 12. A 24V UPS needs 2 batteries, 72V needs 6, and 192V needs 16 — wired in series. How long it runs is then a separate decision, set by each battery's Ah and your load.
The count is set by DC system voltage
An online UPS runs its inverter from a DC bus at a fixed voltage, and you reach that voltage by wiring 12V batteries in series. So the count is simply the DC system voltage divided by 12 — and it is decided by the model you buy, not by how much backup you want. A 72V unit will always want six 12V batteries in its string; you cannot run it on five or seven.
The rules
- Batteries needed = DC system voltage ÷ 12 (e.g. 72V ÷ 12 = 6 batteries).
- DC voltage rises with capacity: ~24–36V at 1 kVA, 72V at 2–3 kVA, 192V at 5.5–10 kVA.
- Series wiring sets the voltage (the count); Ah sets the runtime.
- Every battery in a series string must be identical and replaced as a matched set.
- More runtime = higher-Ah batteries (or a parallel identical string), not extra 12V blocks in series.
DC bus to battery count, at a glance
| Capacity | Typical DC system | 12V batteries in series |
|---|---|---|
| 1 kVA | 24V (or 36V) | 2 (or 3) |
| 2–3 kVA | 72V | 6 |
| 5–5.5 kVA | 192V | 16 |
| 6–7.5 kVA | 192V | 16 |
| 10 kVA | 192V | 16 |
Typical Microtek single-phase online UPS — battery count by DC system
How long will it run?
Runtime comes from the total energy in the bank (Ah × voltage) measured against your load, minus conversion losses. The small SMF batteries built into a UPS hold little energy, so they give the familiar few minutes — enough for a clean shutdown. Real runtime comes from an external bank of higher-Ah VRLA batteries; double the Ah and you roughly double the backup at the same load.
Backup Time Calculator
InteractiveEstimate how long your battery will keep your load running during a power cut.
Estimated backup
3 h 4 min
At this steady load.
Usable energy
918 Wh
1 × 12V × 150Ah × 60% × 85%
If load doubles
1 h 32 min
Backup roughly halves as load rises.
Assumes ~85% inverter efficiency and usable depth-of-discharge of 60% (lead-acid tubular/flat). Real backup depends on battery age, temperature and the exact load running at the time — a tired 3-year-old battery can give 30-40% less. For an exact figure for your home, talk to our team.
Estimate runtime from a battery bank at your load
Sizing the bank for a target runtime
Work backwards from the hours you want: the bank's energy needs to cover your load in watts times those hours, with margin for losses and battery ageing. The UPS fixes the voltage (and so the battery count); you choose the Ah to hit the runtime. We do this calculation with you and supply the matched set.
Battery Bank Sizer
InteractiveWork out the battery capacity you need for a target number of backup hours.
Capacity needed (at 12V)
261 Ah
3137 Wh of battery.
Suggested bank
2 × 150Ah
Round up to whole batteries.
Reality check
Sensible
A clean, practical bank size.
Sizes for usable energy after depth-of-discharge and ~85% inverter losses. Going one size up adds headroom for ageing and cold mornings. Batteries in series raise the bank voltage (24V/48V) at the same Ah; in parallel they add Ah at 12V — our team will wire it correctly for your inverter.
Size the battery bank Ah for your target backup
Why matched, and why not just add more
In a series string the weakest battery sets the backup for the whole set, so all of them must be the same make, Ah and age, and they are replaced together — not one at a time. To extend runtime you raise the Ah, or add a second identical string in parallel; you never just add odd batteries in series, because that changes the voltage the UPS expects. Our guide to finding the weak battery in a series bank explains why a single tired unit drags the whole bank down.
Where to next
Frequently Asked Questions
How many batteries does a 3kVA online UPS need?
A 3 kVA single-phase Microtek online UPS typically runs on a 72V DC system, which means six 12V batteries wired in series. Always confirm your model's DC system voltage and divide it by 12 to be sure.
Can I add more batteries to get longer backup?
Not more in series — that string is fixed by the UPS's DC voltage. For longer runtime you use higher-Ah batteries, or add a second identical string in parallel. The count stays the same; the Ah goes up.
How long will an online UPS run on its batteries?
On the small internal batteries, usually a few minutes — enough for a safe shutdown. With an external higher-Ah VRLA bank it can run for an hour or more, depending on your load, the Ah and the battery's age.
Need help choosing?
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