Mobile & Electronics

UPS Backup Calculator

Roughly how long a UPS keeps a load running, from its battery rating and what you have plugged into it.

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Your figures

Small desktop units are usually 12 V; larger ones use 24 V or 48 V banks.

From the battery inside the UPS. Small units commonly use 7 to 9 Ah sealed batteries.

Everything plugged into the battery-backed sockets, added up.

Leave blank for this page's 80% assumption, which covers the inverter's loss.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A desktop UPS gives you minutes, not hours, and that is the point of it. It exists so you can save your work and shut down cleanly, not so you can keep working through a power cut.

A typical small UPS with a 12 V 7.2 Ah battery holds about 86 Wh. Running a 150 W desktop and monitor, that is around twenty-eight minutes at 80% efficiency — and rather less in practice, because the battery is probably not new.

How to use this calculator

  1. Find the battery inside the UPS, or in its specifications. Small units use sealed lead-acid batteries, commonly 12 V and 7–9 Ah.
  2. Add up everything plugged into the battery-backed sockets. Printers and other high-draw devices should not be among them.
  3. Leave the efficiency at the page's 80% assumption unless you know better.

The formula

Battery energy = Voltage x Capacity
Usable energy  = Battery energy x Efficiency
Backup time    = Usable energy / Load

The efficiency covers the inverter's conversion loss. It does not cover the fact that a sealed lead-acid battery discharged hard delivers well below its rated capacity — a battery rated over 20 hours being emptied in 20 minutes gives substantially less than the label. That is why the real figure tends to fall short of this one.

SymbolMeaningUnit
V UPS battery voltage V
Ah UPS battery capacity Ah
Load Total connected load W
Efficiency Inverter efficiency %

Worked example

A 12 V 7.2 Ah UPS battery running a 150 W desktop at 80%:

Energy  = 12 x 7.2      = 86.4 Wh
Usable  = 86.4 x 0.80   = 69.1 Wh
Backup  = 69.1 / 150    = 0.46 h = about 28 minutes

Halve the load and you roughly double the time, which is why unplugging the monitor during a cut buys you noticeably longer.

Notes and limits

  • This is an estimate from your figures. It is not a manufacturer's runtime chart, which is measured on their own hardware with their own batteries and is the number to trust if you have it.
  • Heavy discharge costs capacity. A battery rated over 20 hours delivers materially less when drained in twenty minutes, so expect less than this figure rather than more.
  • UPS batteries age quickly, particularly in the heat. Three to five years is typical, and a tired battery can give a fraction of its original backup with no warning.
  • Do not run a laser printer from a UPS. Its warm-up draw is enormous and it will either trip the unit or flatten it immediately.
  • Test it occasionally. A UPS that has never been tested is a UPS whose battery condition nobody knows.

What this calculation assumes

  • The load is steady and everything listed is on the battery-backed sockets.
  • Efficiency covers the inverter only, not the capacity lost to heavy discharge.
  • The battery holds its rated capacity, which an older one will not.
  • This is an estimate from your figures, not the manufacturer's measured runtime.

Frequently asked questions

How long does a UPS last during a power cut?

For a typical desktop UPS with a 12 V 7.2 Ah battery and a 150 W load, roughly 20 to 30 minutes when new - and less as the battery ages. UPS units are designed for a clean shutdown rather than for working through an outage.

Why does my UPS give less backup than it used to?

Battery age, almost always. Sealed lead-acid batteries lose capacity steadily and are usually due for replacement after three to five years, sooner in a hot room.

Can I fit a bigger battery to get longer backup?

Some UPS models support external battery packs and are designed for it. Fitting a larger battery to a unit not built for it can overload the charging circuit. Check the manufacturer's documentation rather than assuming.

Should my printer be on the UPS?

No. A laser printer's warm-up draw is very large and will trip or flatten a typical desktop UPS. Plug it into a surge-protected socket that is not battery-backed.

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