Mobile & Electronics

Battery Energy Calculator

What a battery holds and what you actually get out of it, with the conversion loss shown as its own line.

5 inputs Free, no sign-up

Your figures

The figure printed on the battery or in its specifications.

From the battery label. It is not assumed for you.

Leave blank to use this page's assumption of 90%, which the result names. Enter a lower figure for anything that converts the voltage, such as a power bank.

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 battery holds one amount of energy and delivers a smaller one. The gap is conversion loss, and how big it is depends on what the energy passes through on its way out — a direct DC load loses almost nothing, a power bank stepping 3.7 V up to 5 V loses a third.

This shows both figures with the loss between them, so the assumption is visible rather than baked into a single number.

How to use this calculator

  1. Enter the capacity and the battery voltage from its label.
  2. Set an efficiency if you know it. Leave it blank and the page uses 90% and says so.
  3. Read both figures. Nominal is arithmetic; usable is an estimate, and the difference is the efficiency you chose.

The formula

Nominal energy = Voltage x Capacity
Usable energy  = Nominal energy x Efficiency

Nominal energy is what the battery holds and can be worked out exactly. Usable energy depends on where it is going: a load wired directly to the battery sees almost all of it, while anything that converts the voltage — a power bank's 5 V output, an inverter's mains output — loses a share as heat.

For lead-acid there is a second reduction that is not efficiency at all: the battery should not be discharged much past half, so usable energy is lower again regardless of conversion.

SymbolMeaningUnit
V Battery voltage V
Capacity Charge held Ah
Efficiency Share that reaches the load %
Wh Energy Wh

Worked example

A 5,000 mAh phone battery at 3.7 V, at 90% efficiency:

Nominal = 5000 x 3.7 / 1000 = 18.5 Wh
Usable  = 18.5 x 0.90        = 16.65 Wh

The same battery through a power bank's 5 V converter at 65%:

Usable = 18.5 x 0.65 = 12.03 Wh

Notes and limits

  • Nominal energy is exact arithmetic on the rating. Usable energy is an estimate and always carries the efficiency that produced it.
  • There is no universal efficiency. A direct DC load is near 95%; a power bank stepping up to 5 V is commonly 60–70%; an inverter making mains is around 80–85%.
  • Lead-acid batteries have a separate limit: discharging past about half repeatedly shortens their life sharply, so plan on half the nominal energy regardless of conversion.
  • Batteries lose capacity with age and deliver less in the cold. A pack several years old holds noticeably less than its label says.
  • These calculators are informational. Do not modify batteries, bypass protection circuits, or use a cell that is swollen, damaged or hot.

What this calculation assumes

  • Nominal energy is exact; usable energy is an estimate at the efficiency shown.
  • Efficiency is your figure, or this page's stated assumption when you leave it blank.
  • No allowance is made for battery age, temperature, or lead-acid depth-of-discharge limits.

Frequently asked questions

What is the difference between nominal and usable battery energy?

Nominal is what the battery holds - voltage times capacity. Usable is what reaches the load after conversion losses. The gap depends on what the energy passes through on the way out.

What efficiency should I use?

It depends on the path. A load wired straight to the battery loses very little, so 90-95%. A power bank converting to 5 V is commonly 60-70%. An inverter producing mains is around 80-85%. This page uses 90% when you leave it blank, and says so.

Why does my battery seem to hold less than it used to?

Lithium batteries lose capacity with cycles and with age, and deliver less in the cold. A pack a few years old holding 80% of its original capacity is normal rather than faulty.

Can I use the full energy of a lead-acid battery?

No. Discharging one below about half its capacity repeatedly shortens its life sharply. Size for roughly twice the energy you need, or fold that into the efficiency figure.

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