Mobile & Electronics

Battery Backup Calculator

How long a battery of a given voltage and capacity runs a given load, with the conversion loss shown rather than hidden.

7 inputs Free, no sign-up

Your figures

12 V for a typical inverter or vehicle battery; 24 V or 48 V for a series bank.

The figure printed on the battery or in its specifications.

What the battery has to run, in watts. It is on the rating plate of whatever you are powering.

Leave blank for this page's 90% assumption. Lower it to cover an inverter's losses or a lead-acid battery's usable depth.

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

Backup time is usable energy divided by load. A 12 V 100 Ah battery holds 1,200 Wh; at 90% efficiency that is 1,080 Wh usable, which runs a 200 W load for about five and a half hours.

The figure that decides the answer is the load, and it is also the one people are least sure about. A guess that is 50 W out on a 200 W load moves the answer by more than an hour.

How to use this calculator

  1. Enter the battery voltage and capacity from its label.
  2. Enter the total load in watts — add up the rating plates of everything that will run.
  3. Set an efficiency. 90% suits a DC load wired straight to the battery; for an inverter producing mains, 80% is closer, and for lead-acid you may want lower still to allow for depth of discharge.

The formula

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

Efficiency here does double duty. It covers the conversion loss, and it is also where to fold in the fact that a lead-acid battery should not be run flat — if you only intend to use half of one, an efficiency of 40–45% gives a realistic answer for a typical inverter.

SymbolMeaningUnit
V Battery voltage V
Ah Battery capacity Ah
Efficiency Share reaching the load %
Load Power being drawn W

Worked example

A 12 V 100 Ah battery running a 200 W load at 90%:

Energy  = 12 x 100      = 1,200 Wh
Usable  = 1200 x 0.90   = 1,080 Wh
Backup  = 1080 / 200    = 5.4 hours

The same battery through an inverter, with lead-acid depth allowed for at 45%:

Usable = 1200 x 0.45 = 540 Wh
Backup = 540 / 200   = 2.7 hours

Half the answer, and the more realistic one for an inverter installation.

Notes and limits

  • An estimate, not a guarantee. Real backup runs shorter: losses rise under heavy load, a battery delivers less as it ages and in the cold, and a varying load draws more at its peaks.
  • Lead-acid should not be discharged much past half. Either halve the answer or fold it into the efficiency, but do not plan on the full nominal energy.
  • Lead-acid capacity is rated at a slow discharge, usually over 20 hours. Drawing harder gives less — sometimes substantially less — which this calculation does not model.
  • Lithium batteries tolerate deeper discharge and hold their voltage better under load, so the nominal figure is closer to the truth for them.
  • Add up every load that will be running, not just the one you are thinking about.

What this calculation assumes

  • The load is steady at the figure entered.
  • Efficiency is your figure or this page's stated assumption.
  • No allowance is made for discharge rate, battery age or temperature.

Frequently asked questions

How long will a 100 Ah battery last?

It depends entirely on the load and the voltage. A 12 V 100 Ah battery holds 1,200 Wh, so at 200 W it lasts about 5.4 hours at 90% efficiency - or roughly half that through an inverter with lead-acid depth allowed for.

What efficiency should I use?

For a DC load wired directly, 90-95%. For an inverter producing mains, 80-85%. For a lead-acid battery you only intend to half-discharge, around 40-45% gives a realistic combined figure.

Why does my battery last less than calculated?

Usually depth of discharge, load higher than estimated, age, or the discharge rate. Lead-acid rated over 20 hours delivers noticeably less when drawn down in two.

Can I run the battery completely flat?

Not a lead-acid one - doing so repeatedly shortens its life sharply, and once badly can ruin it. Lithium tolerates deeper discharge but still has a lower limit its protection circuit enforces.

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