Mobile & Electronics

Power Bank Energy Calculator

What a power bank really delivers against the number on the case, and why the two are so far apart.

5 inputs Free, no sign-up

Your figures

The headline figure printed on the case. It is the cell capacity, not what comes out of the USB port.

The voltage the capacity is quoted at. Power bank cells are usually 3.6 V or 3.7 V nominal; check the small print on the case.

Leave blank for this page's assumption of 65%, which the result names. Good packs reach 70%; cheap ones fall below 60%.

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

The number on a power bank is not what you get. A 20,000 mAh pack holds about 74 Wh of energy at its cell voltage, and delivers somewhere around 48 Wh at the USB port — because stepping 3.7 V up to 5 V costs energy, and every conversion loses some as heat.

That is not a defect and it is not false advertising: the case is honestly labelled with the cell capacity. It just is not the figure that tells you how many times your phone will charge.

How to use this calculator

  1. Enter the capacity printed on the case.
  2. Enter the cell voltage — it is in the small print, usually 3.6 V or 3.7 V.
  3. Set an efficiency if you know your pack's, or leave it blank for the page's 65% assumption.
  4. Compare the nominal and usable figures. The second is what to plan around.

The formula

Nominal energy = Capacity (Ah) x Cell voltage
Usable energy  = Nominal energy x Efficiency

Two things reduce the headline number. The first is that mAh at 3.7 V is fewer mAh at 5 V, for the same energy — that alone takes 20,000 mAh down to about 14,800. The second is conversion loss in the step-up circuit, which takes it down again.

Both together explain why a 20,000 mAh pack typically delivers around 12,000–13,000 mAh at 5 V. The energy figure in watt-hours is the honest way to think about it, because it does not change with the voltage.

SymbolMeaningUnit
Capacity Cell capacity from the case mAh
V Cell voltage V
Efficiency Share surviving the conversion %

Worked example

A 20,000 mAh power bank at 3.7 V, at 65% efficiency:

Nominal = 20000 x 3.7 / 1000 = 74 Wh
Usable  = 74 x 0.65           = 48.1 Wh

Against a 5,000 mAh 3.7 V phone (18.5 Wh), that is about 2.6 full charges — not the four the headline numbers suggest.

Notes and limits

  • The case is labelled with cell capacity, which is honest but not what reaches your phone.
  • Efficiency varies with the pack, the output current and the temperature. 60–70% is the usual range; this page assumes 65% and lets you change it.
  • Fast charging tends to be slightly less efficient than slow charging, because higher currents mean more loss as heat.
  • Airlines limit lithium batteries by watt-hours, commonly 100 Wh in cabin baggage without approval. The nominal figure here is the one they mean.
  • For how many charges of a particular phone, use the power bank backup calculator.

What this calculation assumes

  • The capacity on the case is the cell capacity at the cell voltage, which is how packs are labelled.
  • Efficiency is your figure, or this page's 65% assumption when left blank - not a specification.
  • Real output varies with current, temperature and the age of the pack.

Frequently asked questions

Why does my 20,000 mAh power bank not give 20,000 mAh?

Two reasons. The cells are at about 3.7 V and the output is 5 V, so the same energy is fewer mAh at the higher voltage. On top of that, the step-up conversion loses energy as heat. Expect roughly 12,000 to 13,000 mAh at 5 V from a 20,000 mAh pack.

Is my power bank faulty if it delivers less than the label?

Almost certainly not. Delivering 60-70% of the labelled capacity at 5 V is normal and expected. Well below 60% suggests a cheap pack or an ageing one.

What efficiency do power banks have?

Usually 60-70%, varying with the pack, the output current and the temperature. There is no universal figure, which is why this page treats 65% as an assumption you can change rather than as a specification.

Can I take a power bank on a plane?

Airlines generally allow lithium batteries up to 100 Wh in cabin baggage without approval, and require them in the cabin rather than the hold. A 20,000 mAh pack at 3.7 V is 74 Wh, comfortably under. Check your airline, since rules differ.

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