Power bank calculator
Tell us how many phones you need to keep going and for how long. We’ll work out the power bank capacity you need, allowing for charging losses.
Your result
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How the calculator works
- We assume a typical modern phone battery holds about 15 Wh.
- We multiply by the number of phones, charges a day and days.
- We allow for the 30–40% of energy lost when a power bank charges a phone.
- We suggest the smallest power bank that covers it. If you’d need lots of power banks, we suggest a small power station instead – it’s usually better value and charges several phones at once.
Phone charges by power bank size
| Power bank | Capacity | Typical phone charges |
|---|---|---|
| 10,000 mAh | 37 Wh | about 1½ |
| 20,000 mAh | 74 Wh | about 3 |
| 26,800 mAh | 99 Wh | about 4 |
Larger phones and older batteries will get fewer charges.
Making your phone battery last
- Turn on low power mode and turn down the screen brightness.
- Close apps you don’t need and turn off Bluetooth if you aren’t using it.
- Use text messages rather than calls where you can – they use less battery and often get through when networks are busy.
- Keep a printed list of important numbers in case your phone runs out.
Frequently asked questions
How many phone charges does a 20,000 mAh power bank give?
Roughly three full charges of a typical modern smartphone. A 20,000 mAh bank holds about 74 Wh, around 65% of that reaches the phone after conversion losses, and a typical phone battery holds about 15 Wh.
How do I convert mAh to Wh?
Multiply the mAh rating by 3.7 (the nominal voltage of a lithium-ion cell) and divide by 1,000. So 10,000 mAh × 3.7 ÷ 1,000 = 37 Wh. Many power banks print the Wh figure on the label too.
Why don’t I get the full capacity out of a power bank?
Energy is lost converting the battery’s voltage up to USB voltage, in the cable, and in the phone’s own charging circuit. In practice, expect around 60–70% of the rated capacity to end up in your phone.
Can I take a power bank on a plane?
Most airlines allow power banks under 100 Wh in hand luggage without approval, but not in checked baggage. Rules vary, so check with your airline.
How often should I recharge a power bank I keep for emergencies?
Power banks slowly lose charge in storage. Topping them up every three months or so, and checking them when you check the rest of your kit, means they’re ready when needed.
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General guidance only – not a substitute for official or medical advice. See every figure we use and where it comes from.