1. Home
  2. All tools
  3. Power outage and stockpile calculators
  4. Power bank and portable power station calculator
Free · No sign-up · Works offline

Power bank and portable power station calculator

Work out the number of charges and the runtime from the rated capacity in mAh or Wh. The conversion efficiency is an assumption you can change.

How many phone charges are left in a power bank

mAh
The "10000mAh" printed on the case. That number is measured at the cell voltage of about 3.7V, not at the 5V USB output.
mAh
You can find this on the specification page for your handset.
%
What is lost stepping 3.7V up to 5V, plus heat. This varies with the model, the temperature, the cable and the charging speed, so replace it with the figure in your own specification sheet where one is given.
Estimated number of charges
—
These figures are reference estimates based on general rules of thumb. The amount or time you actually need varies with your household, your equipment and the temperature. Always check the guidance published by your local government and by national authorities.

How long a portable power station runs an appliance

Wh
W
%
Efficiency tends to drop at very low loads, and USB output is usually better than AC output. Use the figure from your specification sheet where one is given.
Estimated runtime
—
These figures are reference estimates based on general rules of thumb. The amount or time you actually need varies with your household, your equipment and the temperature. Always check the guidance published by your local government and by national authorities.

Working backwards: how many Wh do I need

W
hours
This uses the efficiency you entered above.
Estimated capacity needed
—
These figures are reference estimates based on general rules of thumb. The amount or time you actually need varies with your household, your equipment and the temperature. Always check the guidance published by your local government and by national authorities.

Appliances that cycle on and off, such as a fridge

A refrigerator runs its compressor intermittently, so using the rated wattage makes the runtime look far shorter than it really is. If the catalogue gives an annual electricity consumption in kWh per year, you can work out an average power draw from it.

kWh/year
Printed on the energy label or in the catalogue of fridges, air conditioners and similar appliances.
Estimated average power draw
—
Calculated as annual consumption [kWh/year] x 1000 / 365 / 24. The real figure moves a lot with the season, how often the door is opened and how full the appliance is.
These figures are reference estimates based on general rules of thumb. The amount or time you actually need varies with your household, your equipment and the temperature. Always check the guidance published by your local government and by national authorities.

Add it to your home screen and it opens offline

Once you have opened this page it is stored on your device and starts even with no connection. You cannot search for a page while you are out of signal, so add it to your home screen from your browser menu while you still can. Ads and analytics do not load offline, but every calculation runs on your device, so nothing breaks.

Saved on this device. It will open without a connection.

How to use this tool

This page estimates how many times the battery in your hand can charge a phone, and how long a power station can run an appliance. It is useful when the power is out, and equally useful when you are comparing capacities before buying one.

Why you cannot simply divide the mAh figures

The "10000mAh" on a power bank is measured at the lithium-ion cell voltage of roughly 3.7V, while USB delivers 5V. Dividing one mAh figure by another compares two different voltages, so this tool converts both sides into watt-hours first. Boost conversion and heat then take a further share.

Efficiency is an assumption you can adjust

Efficiency depends on the model, the temperature, the charging speed and the cable, and there is no single settled value. It is therefore an input field, and the result also shows what you would get at 60-70% (power bank) and at 70% and 90% (power station). Replace it with the figure from your own specification sheet whenever one is published.

Enough capacity does not always mean it will run

Microwave ovens, hair dryers and electric kettles draw a surge above their rated wattage when they start. If the rated and peak output in watts of the power station is too low, the appliance will not run even when there is plenty of capacity in watt-hours. Check the output rating, not just the capacity.

Frequently asked questions

Can I just divide the power bank mAh by my phone mAh?
+
No. The rated mAh of a power bank is measured at the 3.7V cell voltage, while USB output is 5V. Because the voltages differ, both sides have to be converted into watt-hours first, and boost conversion and heat cause further losses.
What efficiency should I enter?
+
Use the figure from your own specification sheet if it gives one. If not, keep the default and read the range shown next to it as the likely spread. Efficiency changes with model, temperature and charging speed, so we do not present a single value as fact.
Why does the runtime for a fridge look too short?
+
Because a fridge cycles its compressor on and off. The rated wattage applies only while the compressor runs, so using it directly makes the runtime look short. This page has a field that converts the catalogue kWh-per-year figure into an average power draw.
Why will an appliance not run even though capacity is left?
+
Microwave ovens, hair dryers and kettles draw a surge above their rated wattage at start-up. If the rated or peak output of the power station is too low, the appliance will not run even when plenty of capacity remains. Check the output rating as well as the capacity.
Is anything I type sent anywhere?
+
No. Every calculation runs inside your browser, and nothing you type is sent to or stored on a server. That is also why the tool behaves the same way with no connection.
Formulas
・Stored energy [Wh] = rated capacity [mAh] x 3.7 [V] / 1000
・Charges = stored energy [Wh] x efficiency / (phone capacity [mAh] x 3.7 [V] / 1000)
・Runtime [h] = capacity [Wh] x efficiency / power draw [W]
・Capacity needed [Wh] = power draw [W] x hours [h] / efficiency
・Average power [W] = annual consumption [kWh/year] x 1000 / 365 / 24
・Efficiency is not a fixed constant. It is an assumption you can edit (defaults: 65% for a power bank, 80% for a power station).
Sources (accessed 2026-07-28)

These are the sources for the figures used here. The numbers are based on published guidance; we do not reproduce the text, tables or illustrations of those pages. Follow the links for the full explanation.

About the efficiency figure: it is not quoted from any published value. It is designed as an assumption you enter yourself. To avoid presenting a single number as fact, the results also show what you would get at 60-70% (power bank) and at 70% and 90% (power station).

Primary sources are updated from time to time. Please check the linked pages for the latest wording before making any important decision. We link to public-sector material rather than reproducing it.

Before you use this tool
The figures shown here are reference information produced by general formulas. They are not guaranteed to be accurate, complete or safe. This site does not represent any government body, manufacturer or other organisation, and accepts no liability for any loss arising from the use of, or inability to use, this tool. Always check the manual for your equipment and the guidance published by the relevant authorities before acting. See our Terms and Disclaimer for details.