When the power goes off, your phone becomes your torch, your news source and your way to reach family, so it drains fast. GOV.UK's Be Ready checklist includes a charged power bank to charge phones or compatible medical devices, and its storm advice is to charge phones, power banks and medical equipment batteries when a storm is forecast. The question is how big it needs to be.
Checklist: power for 3 days
- Power bank sized for one charge per phone per day (our estimate)
- Kept charged: top it up every few months
- The right cables for every phone (USB-C, Lightning)
- Back-up batteries for any medical equipment, from the supplier
- Phones on low-power mode; use them sparingly
- Torch and radio on their own batteries, not the phone
The simple formula
These are our planning figures, the same ones the calculator uses:
- One full charge per phone per day, with careful use.
- About 4,500 mAh per charge, a typical modern smartphone battery.
- 65% real-world efficiency: you get roughly two-thirds of the number printed on the power bank into your phone.
Rated mAh needed = phones × days × 4,500 ÷ 0.65. For one phone over 3 days that's about 21,000 mAh. For two phones, about 42,000 mAh.
| Phones | Days | mAh needed | Calculator suggests |
|---|---|---|---|
| 1 | 1 | ~6,900 | 1 × 10,000 mAh |
| 1 | 3 | ~20,800 | 1 × 26,800 mAh |
| 2 | 3 | ~41,500 | 1 × 26,800 + 1 × 20,000 mAh |
| 3 | 3 | ~62,300 | 2 × 26,800 + 1 × 10,000 mAh |
| 4 | 3 | ~83,100 | 1 × 26,800 + 3 × 20,000 mAh |
| 2 | 7 | ~96,900 | 3 × 26,800 + 1 × 20,000 mAh |
| 4 | 7 | ~193,800 | 1 portable power station, about 1,000 Wh |
All figures are our estimates. We count phones for adults and children aged 12 and over.
Why you don't get the full mAh
A power bank's mAh rating describes its internal cells at their own low voltage, typically around 3.6 to 3.7 volts. Phones charge at 5 volts or more over USB, and converting between the two, plus heat in the cable and phone, loses energy. Older or cheaper banks lose more, and capacity also fades with age. That's why we plan on about 65% rather than 100%. If yours gives more, you have a margin.
mAh vs Wh
Watt-hours (Wh) measure energy directly, so they're easier to compare across devices. A rough conversion for power banks is Wh ≈ mAh × 3.7 ÷ 1,000. So a 10,000 mAh bank is about 37 Wh, a 20,000 mAh bank about 74 Wh and a 26,800 mAh bank about 99 Wh. A phone charge of 4,500 mAh is about 17 Wh. Portable power stations are usually labelled in Wh.
When a power station makes more sense
Once a household needs more than about four large power banks, which happens with several phones over a week, we suggest one portable power station instead. Our sizing is phone charges × 17 Wh ÷ 0.85 (power stations lose less, by our estimate), rounded up to a common size: 300, 500, 1,000 or 2,000 Wh. Four phones for 7 days is 28 charges, about 560 Wh, so a 1,000 Wh unit. They're also useful for a router, a lamp or small medical devices, but check the device's power needs and the station's instructions first.
A petrol or diesel generator is a different thing. The NHS lists portable generators among the appliances that can produce carbon monoxide: never run one indoors, in a garage or near windows.
Make the charge last
- GOV.UK's phone outage advice is to use devices sparingly to conserve battery until you can charge them again.
- Use a proper torch rather than your phone torch, which GOV.UK notes drains the battery more quickly.
- Get news from a battery or wind-up radio instead of streaming. See our radio guide.
- Turn on low-power mode, lower the brightness and close apps you don't need. (Our suggestion.)
Keep it ready
A power bank is no use flat. Our suggestion: top it up every three months and whenever a storm is forecast, store it at room temperature, and keep the cables with it. If someone at home relies on powered medical equipment, ask the supplier about the right back-up battery and how long it lasts, keep the instructions with your documents as GOV.UK suggests, and look at the Priority Services Register.
Questions people ask
What size power bank do I need for a power cut?
Using our estimate of one full phone charge a day, about 4,500 mAh per phone and 65% real-world efficiency, one phone for 3 days needs about 21,000 mAh of rated capacity, and two phones need about 42,000 mAh.
Why does a 10,000 mAh power bank not give two full charges?
The mAh on the label is measured at the battery cell's own voltage. Converting to the phone's charging voltage and heat losses mean you typically get noticeably less. We plan on about 65%.
Should I use a petrol generator in a power cut?
Never run one indoors or in an enclosed space. The NHS lists portable generators among the appliances that can produce carbon monoxide.
Sources
- GOV.UK Be Ready checklist
- GOV.UK Be Ready: storms
- GOV.UK Be Ready: phone and broadband outages
- GOV.UK Be Ready: power cuts
- NHS: carbon monoxide poisoning
Official advice checked on 29 October 2026. Anything marked as our suggestion or estimate is Survivalify guidance, not an official figure. Survivalify is not part of the government.