Computers & Laptops

UPS Battery Capacity Calculator

The battery capacity in ampere-hours a given load and backup time would need.

4 inputs Free, no sign-up

Your figures

Everything the UPS has to keep running - a computer and a monitor is the usual pair. Measure it with a plug-in meter if you can.

Long enough to save your work and shut down cleanly is the usual target. Hours of backup is an inverter question rather than a UPS one.

Printed on the battery. A single UPS battery is usually 12 V; two in series is 24.

Leave blank for this page's 80% assumption, which the result names.

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

Work backwards from the time you need. A load, a duration and a battery voltage give the capacity, and the capacity is what you buy.

How to use this calculator

  1. Enter the load the UPS has to carry, in watts.
  2. Enter how many minutes of backup you want.
  3. Enter the battery voltage the system runs at - 12 V for a single battery, 24 for two in series.

The formula

Required Ah = Load x Time / (Voltage x Efficiency)

Time is in hours, so fifteen minutes is 0.25. The efficiency covers the inverter's losses and the depth of discharge a lead-acid battery should be held to.

SymbolMeaningUnit
Load What has to keep running W
Time Backup wanted min
Voltage Battery system voltage V
Efficiency Share reaching the load %

Worked example

300 W for fifteen minutes at 12 V, 80% efficient:

Required = 300 x 0.25 / (12 x 0.80) = 7.8 Ah

Which a common 9 Ah desktop battery covers, with a little in hand.

Notes and limits

  • Buy above the figure rather than exactly at it. A lead-acid battery repeatedly taken to empty will not last, and the arithmetic leaves nothing in reserve.
  • Whether your UPS will take a larger battery is a separate question. Many sealed desktop units accept one specific size, and a bigger battery also takes longer to recharge than the unit's charger may be designed for.
  • Lead-acid batteries deliver less than their rating at high discharge rates, so a battery sized exactly to this figure will fall short in practice.
  • For hours rather than minutes of backup, an inverter with a larger battery bank is the right answer - the inverter battery backup calculator covers that.

What this calculation assumes

  • Efficiency defaults to 80% and the result names it as an assumption.
  • The battery is assumed to deliver its full rated capacity, which it will not at this discharge rate.
  • This is the bare figure. Sizing above it is the usual practice and this page does not do it for you.

Frequently asked questions

What battery do I need for 15 minutes of backup?

Multiply the load by the time in hours and divide by the voltage times the efficiency. 300 W for 15 minutes at 12 V and 80% needs about 7.8 Ah, so a 9 Ah battery with something in hand.

Can I fit a bigger battery to my UPS?

Some units are built for it and many are not. A sealed desktop unit usually takes one size, and its charger is matched to that - a much larger battery may never fully recharge between outages.

Why size above the calculated figure?

Because the arithmetic assumes the battery delivers its full rating, which it does not at a UPS's discharge rate, and because a battery run flat repeatedly fails early.

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