Computers & Laptops

Computer UPS Runtime Calculator

How many minutes a UPS will hold your computer up, from its battery and the load it is carrying.

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Your figures

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

From the label on the battery, in ampere-hours.

Batteries in parallel multiply the capacity and the runtime. Two in series double the voltage instead - enter 24 V and one battery for that.

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.

Leave blank for this page's 80% assumption, which the result names. It covers the inverter's losses and the fact that a lead-acid battery should not be run flat.

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 is sized in minutes, not hours. Its job is to keep the machine alive long enough to save the work and shut down properly, and five to fifteen minutes is a unit doing exactly what it was built for.

Enter the battery and the load and this gives the estimate — and says why the real figure will be lower.

How to use this calculator

  1. Read the voltage and capacity from the battery inside the UPS. A single 12 V 7 Ah or 9 Ah battery is the common desktop arrangement.
  2. Say how many batteries there are if the unit has more than one in parallel.
  3. Enter what the UPS is carrying in watts, measured at the desk if you can.

The formula

Battery energy = Voltage x Capacity
Usable energy  = Battery energy x Efficiency
Runtime        = Usable energy / Load

The efficiency covers the inverter's losses and the depth of discharge a lead-acid battery should be held to. It is an assumption, and the result names it.

SymbolMeaningUnit
Voltage Battery voltage V
Capacity Battery capacity Ah
Load What the UPS is carrying W
Efficiency Share reaching the load %

Worked example

A 12 V 9 Ah battery at 80% efficiency, carrying 250 W:

Energy  = 12 x 9      = 108 Wh
Usable  = 108 x 0.80   = 86.4 Wh
Runtime = 86.4 / 250    = 0.35 h = 21 minutes

Expect rather less in practice, for the reasons below.

Notes and limits

  • Lead-acid batteries deliver less than their rating at high discharge rates, and a UPS runs at exactly that. The real figure is commonly well under the arithmetic.
  • Batteries age. Three to five years is a common life, and a tired battery can give a fraction of its rated backup while still appearing to work.
  • Cutting the load extends the time proportionally. Turning off a second monitor during an outage buys real minutes.
  • Test it rather than trusting it. Pull the plug once a year with the machine running and see what actually happens.
  • To work the other way - a runtime you need, and the battery it would take - use the UPS battery capacity calculator.

What this calculation assumes

  • Efficiency defaults to 80% and the result names it as an assumption.
  • The battery is treated as holding its rated capacity, which an aged one does not.
  • Lead-acid capacity loss at high discharge rates is not modelled, so the real figure is lower.

Frequently asked questions

How long will my UPS last in a power cut?

Divide the battery's usable energy by the load. A 12 V 9 Ah battery at 80% efficiency carrying 250 W gives about 21 minutes on paper, and typically less in practice.

Why is my real backup time shorter than this?

Three reasons: lead-acid batteries deliver less at high discharge rates, batteries lose capacity as they age, and the inverter's efficiency falls under heavy load. The arithmetic is the optimistic end.

How do I get more backup time?

Reduce the load or increase the battery. Turning off monitors during an outage is the fastest way; a larger external battery is the permanent one, if your unit supports it.

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