Mobile & Electronics

Battery Capacity Calculator

The battery capacity a load and a runtime need, rounded up to something you can actually buy.

7 inputs Free, no sign-up

Your figures

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

How long the load has to keep running.

The voltage the bank will run at. The same energy needs half the amp-hours at 24 V that it needs at 12 V.

Leave blank for this page's 80% assumption, covering an inverter. For lead-acid, use 40-45% so the capacity also allows for not discharging past half.

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

Working backwards from what you need is the right way to size a battery. Start with the load and the hours, and the capacity follows — rather than buying a battery and hoping.

The answer is rounded up, because a battery that comes to 167 Ah is bought as 180 Ah. Rounding down means the load stops before the time is up, which is precisely what the calculation is for.

How to use this calculator

  1. Add up the load you want to run.
  2. Enter how long it has to last, in hours or minutes.
  3. Choose the system voltage.
  4. Set the efficiency. 80% covers an inverter; 40–45% also allows for only half-discharging a lead-acid battery, which is the realistic figure for most Indian installations.

The formula

Energy needed    = Load x Runtime
Battery energy   = Energy needed / Efficiency
Capacity needed  = Battery energy / Voltage

Dividing by the efficiency rather than multiplying is what makes the battery bigger than the raw energy requirement. The battery has to hold more than the load consumes, because some of it never reaches the load.

SymbolMeaningUnit
Load Power to be run W
Runtime How long for h
V System voltage V
Efficiency Share reaching the load %

Worked example

A 200 W load for 4 hours at 12 V, 80% efficient:

Energy needed   = 200 x 4     = 800 Wh
Battery energy  = 800 / 0.80  = 1,000 Wh
Capacity needed = 1000 / 12   = 83.3 Ah   -> buy 90 Ah

The same requirement at 45%, allowing for a lead-acid half-discharge:

Battery energy  = 800 / 0.45  = 1,778 Wh
Capacity needed = 1778 / 12   = 148 Ah    -> buy 150 Ah

Notes and limits

  • Always round up. Batteries are sold in steps and the calculated figure is a floor.
  • For lead-acid, either use an efficiency around 40–45% or double the answer. Discharging one past about half repeatedly shortens its life sharply.
  • Batteries lose capacity with age and in cold weather, so size for the end of the battery's life rather than the start.
  • A higher system voltage needs fewer amp-hours for the same energy and carries less current, which means thinner cable and smaller losses.
  • To check the answer the other way round, put the result into the battery backup calculator.

What this calculation assumes

  • The load is steady at the figure entered for the whole runtime.
  • The efficiency you choose is where conversion loss and usable depth are combined.
  • The result is a floor; battery age, temperature and discharge rate all argue for more.

Frequently asked questions

What size battery do I need?

Multiply your load by the hours you need, divide by the efficiency, then divide by the system voltage. A 200 W load for 4 hours at 12 V and 80% efficiency needs about 83 Ah - buy 90 or 100.

Why is the battery bigger than the energy I need?

Because not all of the battery's energy reaches the load. Conversion loses some, and for lead-acid a further share is off limits because deep discharge damages the battery.

Should I round up or down?

Up, always. A battery below the calculated figure will not last the time you asked for, which is the one thing this calculation is meant to prevent.

Does a higher voltage system need a smaller battery?

Fewer amp-hours, not less energy. 1,000 Wh is 83 Ah at 12 V or 42 Ah at 24 V - the same energy either way, but the higher voltage carries less current, so cabling is thinner and losses lower.

Related calculators