Mobile & Electronics

Battery Load Calculator

What a particular load does to a particular battery - how long it runs and how much energy it takes.

7 inputs Free, no sign-up

Your figures

The figure printed on the battery or in its specifications.

The device or devices the battery is running.

Leave blank to use this page's assumption, which the result names. There is no universal correct figure - it depends on the hardware, the load and the temperature.

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

Adding a load to a battery is a trade you can quantify. Every watt you add takes time off the runtime in direct proportion — halve the load and the battery lasts twice as long, which is the most useful relationship in this whole subject.

This page is for working out that trade: what a particular load does to a particular battery, and what it would take to run it for as long as you want.

How to use this calculator

  1. Enter the battery's voltage and capacity.
  2. Enter the load you want to run.
  3. Read the runtime, then change the load and read it again. The relationship is directly proportional, so the comparison is easy to reason about.

The formula

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

Because runtime is energy divided by load, the two are inversely proportional: doubling the load halves the runtime exactly. That makes the arithmetic of "what if I turn that off" simple — a 50 W device removed from a 200 W load extends the runtime by a third.

SymbolMeaningUnit
V Battery voltage V
Ah Battery capacity Ah
Load Total power drawn W

Worked example

A 12 V 100 Ah battery at 90% efficiency:

Usable energy = 12 x 100 x 0.90 = 1,080 Wh

At  50 W load:  21.6 hours
At 100 W load:  10.8 hours
At 200 W load:   5.4 hours

Each doubling of the load exactly halves the time.

Notes and limits

  • Runtime is inversely proportional to load. That relationship holds exactly in the arithmetic and approximately in reality — heavy discharge costs a little extra capacity on top.
  • Add up every load, including the standby draw of anything left connected.
  • For lead-acid, plan on about half the nominal capacity, or fold it into the efficiency.
  • A load that varies draws more at its peaks than its average, so a steady-load estimate is optimistic for anything with a motor or a compressor.
  • To go the other way — from a load and a target runtime to a battery size — use the required capacity calculator.

What this calculation assumes

  • The load is steady at the figure entered.
  • Effective capacity lost to a fast discharge rate is not modelled.
  • Efficiency is your figure or this page's stated assumption.

Frequently asked questions

How does load affect battery runtime?

Inversely and proportionally. Double the load and the runtime halves; halve the load and it doubles. In practice a heavy discharge costs slightly more than the arithmetic suggests, particularly for lead-acid.

How do I work out my total load?

Add up the wattage of everything connected, from each device's rating plate. Include anything on standby, since a few watts running continuously is not nothing over a long backup.

What happens if I exceed the battery's safe discharge rate?

The battery delivers less than its rated capacity, its voltage sags under the load, and it heats up. Sustained over-discharge damages the battery and shortens its life considerably.

Why does a heavier load give less than proportionally less time?

Because a battery's effective capacity falls as the discharge rate rises - an effect most pronounced in lead-acid. The arithmetic here does not model it, so treat fast-discharge answers as optimistic.

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