In short: As a rough guide, divide the power bank's mAh by your phone's battery mAh, then take off about a third for losses: a 10,000 mAh bank gives a 4,000 mAh phone roughly one and a half full charges. That's our estimate, not an official figure. Be Ready and the British Red Cross both list a power bank.
Key facts
- The British Red Cross lists a mobile phone charger and portable power bank among essential emergency supplies, and fully charged power banks as useful in power cuts. Source
- The British Red Cross advises recharging power banks and replacing used items as part of looking after your kit. Source
- GOV.UK Be Ready says your mobile phone might stop working if your local mast is in the affected area, and that using your phone's torch drains the battery more quickly. Source
- The GOV.UK Be Ready emergency kit checklist includes a power bank. Source
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A charged power bank is on the Be Ready checklist and the British Red Cross kit list for a simple reason: in a power cut, your phone is your torch, your news and your way to call for help. This guide shows how to estimate how far yours will go. It's part of our power cuts hub.
A simple way to estimate charges
You need two numbers, both in mAh:
- Your power bank's capacity. It's printed on the power bank or the box (for example 10,000 mAh).
- Your phone's battery size. Search your phone model plus "battery capacity", or check the maker's specification page.
Then: power bank mAh × 2/3 ÷ phone battery mAh = rough number of full charges.
The "two thirds" is our rule of thumb, not an official figure: some energy is always lost turning the power bank's battery into the charge your phone takes, and older power banks hold less.
| Power bank | Phone battery 3,000 mAh | 4,000 mAh | 5,000 mAh |
|---|---|---|---|
| 5,000 mAh | about 1 | under 1 | under 1 |
| 10,000 mAh | about 2 | about 1½ | about 1¼ |
| 20,000 mAh | about 4 | about 3 | about 2½ |
All figures are estimates using the rule above, not tested results.
Make the battery last
- Don't use the phone as a torch. Be Ready says it drains the battery more quickly. Keep a real torch with spare batteries.
- Turn on low power or battery saver mode, and turn the screen brightness down.
- Text rather than call when you can, and keep calls short.
- Charge one phone at a time and agree who in the household keeps theirs off as a spare.
- Get news from a radio. Be Ready says your mobile might stop working if your local mast is in the affected area. A battery or wind-up FM radio still works; see our wind-up radio guide.
Keep it ready
- Charge the power bank now, and write the date on a label.
- Recharge it when you do your six-monthly kit check, as the British Red Cross advises (more often in storm season).
- Keep the right cable with it.
- Write 105, the free power cut number, on paper, so you don't need the phone to find it.
Limitations
The charge estimates on this page are our own rule of thumb, not official or tested figures; real results vary by phone, power bank, cable and temperature. Follow the maker's instructions for charging and storing lithium batteries.
Turn this guide into a written plan
Reading helps. Writing it down is what makes it work when the phones are flat. Start with the free plan builder, or get the full printable version:
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Frequently asked questions
How many charges does a 10,000 mAh power bank give?
Our rough rule: about two thirds of the rated capacity reaches the phone. So 10,000 mAh gives roughly 6,500 mAh, about one and a half charges for a 4,000 mAh phone battery. Check your own phone's battery size; this is an estimate, not a tested figure.
How long does a power bank keep its charge in a drawer?
It slowly loses charge. The British Red Cross advises recharging power banks as part of looking after your kit, so top yours up when you do your six-monthly kit check, or more often in storm season.
Will my phone work in a power cut?
Often, but not always. Be Ready says your mobile might stop working if your local mast is in the affected area. A battery or wind-up FM radio still works without the phone network.
Sources
- British Red Cross: Prepare an emergency kit (checked 30 Oct 2026)
- GOV.UK Be Ready: Power cuts (checked 30 Oct 2026)
- GOV.UK Be Ready: Checklist PDF (checked 28 Oct 2026)