Mobile & Electronics

Power Bank Backup Calculator

How long a power bank runs a device, and how many charges it gives a phone whose battery you supply.

10 inputs Free, no sign-up

Your figures

The figure printed on the case.

From the small print on the case, usually 3.6 V or 3.7 V.

What the device pulls while charging. A phone on a standard charger is around 5 to 10 W; on a fast charger, more.

Leave blank for this page's 65% assumption, which the result names.

How many charges

Enter this and the phone's battery voltage to see how many full charges you get. Both come from your phone's specifications.

Usually 3.7 V or 3.85 V. Required alongside the capacity - a capacity in mAh without its voltage is not a quantity of energy.

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 power bank's runtime is its usable energy divided by what the device draws. The trap is the word usable: a 20,000 mAh pack holds 74 Wh and delivers around 48 Wh, so the arithmetic has to start from the smaller figure.

Enter your phone's battery and you also get the number people actually want — how many full charges. Both of your phone's figures come from you, because this page does not have a database of phones and would be guessing if it pretended to.

How to use this calculator

  1. Enter the power bank's capacity and cell voltage from the case.
  2. Enter what your device draws. A phone on a standard charger is roughly 5–10 W; a tablet or a fast-charging phone is more.
  3. Adjust the efficiency if you know your pack's, or leave it for the page's 65% assumption.
  4. For a charge count, add your phone's battery capacity and its voltage — both are in its specifications.

The formula

Nominal energy = Capacity (Ah) x Cell voltage
Usable energy  = Nominal energy x Efficiency
Runtime        = Usable energy / Device power
Charges        = Usable energy / Phone battery energy

The charge count is a comparison of two energies, which is why the phone's voltage is needed as well as its capacity. Comparing the two mAh figures directly — 20,000 against 5,000 and concluding four charges — ignores the conversion loss entirely and overstates the answer by about half.

SymbolMeaningUnit
Capacity Power bank cell capacity mAh
Efficiency Share surviving the conversion %
Load What the device draws W

Worked example

A 20,000 mAh power bank at 3.7 V, 65% efficient, running a 5 W device:

Nominal = 74 Wh
Usable  = 74 x 0.65 = 48.1 Wh
Runtime = 48.1 / 5  = 9.6 hours

Charging a 5,000 mAh 3.7 V phone (18.5 Wh) instead:

Charges = 48.1 / 18.5 = 2.6 full charges

Not the four the headline numbers suggest.

Notes and limits

  • An estimate, not a guarantee. Real output falls with heavy load, cold, heat and the age of the pack.
  • Comparing mAh to mAh overstates the charge count by roughly half, because it ignores both the voltage difference and the conversion loss.
  • A partial charge is more useful than the headline count suggests: 2.6 charges means two full ones and most of a third, not two.
  • Fast charging is slightly less efficient, so a pack gives marginally fewer charges when used at its highest output.
  • Do not use a pack that is swollen, damaged or gets hot. These calculators are informational and none of this is advice to modify a battery.

What this calculation assumes

  • The device draws the load entered steadily; charging current actually tapers as a battery fills.
  • Efficiency is your figure or this page's 65% assumption, not a specification.
  • Charge counts compare energies, so the phone's voltage is needed as well as its capacity.

Frequently asked questions

How many times will a 20,000 mAh power bank charge my phone?

For a 5,000 mAh phone at 3.7 V, about 2.6 full charges at a typical 65% efficiency - not the four that dividing the mAh figures suggests. The difference is the voltage step-up and the conversion loss.

Why is it not simply 20,000 divided by 5,000?

Because those mAh figures are at different voltages and because converting between them loses energy. Comparing watt-hours instead of mAh is what makes the calculation honest.

What power draw should I enter?

What the device pulls while charging. A phone on a standard charger is roughly 5-10 W, a tablet 10-20 W, and a fast-charging phone more. If you know the charger wattage your device negotiates, use that.

Does the runtime figure apply to running a device rather than charging one?

Yes - it is energy divided by power either way. For something drawing a steady load, such as a small fan or a light, the runtime figure is the direct answer.

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