What Size Power Bank Do You Need for a 3-Day Power Cut? (UK)

A “charged power bank” is on the government’s Be Ready emergency checklist, launched on 22 October 2026, which asks households to be ready for 3 days of disruption. But the checklist doesn’t say how big. Buy too small and your phone dies on day two. Buy one of the huge, cheap “100,000mAh” banks sold online and you may get far less than it claims. Here’s how to work out the right size for your phones, using the makers’ own figures.

Why a 20,000mAh power bank doesn’t give 20,000mAh

The mAh figure on a power bank is the capacity of its battery cells, measured at their own voltage of about 3.7V. Anker explains that power banks then “convert this energy to 5 V or higher for charging devices, which reduces the usable mAh”. Its rule of thumb is to “expect roughly 60% to 75% of the rated mAh as usable output”.

Anker’s own figures show this in practice:

• Anker says a 20,000mAh power bank will usually charge an iPhone 16 “around 3–4” times, and an iPhone 17 Pro Max “about 2–3” times.
• For its Prime 26K power bank, rated at 26,250mAh, Anker says it delivers “about 15,200 mAh to your devices”, which is about 58%.

So we’ve used 60% in our calculations. It’s at the cautious end of Anker’s range, which suits emergency planning. Real results can be a little lower, so round up when you buy.

How big is your phone’s battery?

For another phone, search for its model name and “battery capacity mAh”. Apple lists the rated capacity on its European spec pages.

What size power bank for each phone

We’ve assumed each phone needs one full charge a day. That’s cautious. Apple’s EU energy label for the iPhone 17, for example, rates it at 41 hours per full charge, and a phone in low power mode uses less. But in a power cut you may lean on your phone more than usual, and cold homes can drain batteries faster.

If your phone starts fully charged, it covers day 1 and the power bank covers days 2 and 3. If it starts nearly flat, the power bank covers all 3 days.

Our calculation: phone battery (mAh) x number of charges, divided by 0.6. These are estimates, not lab tests, so round up when you buy.

Examples for a household

• A couple with an iPhone 16 and a Galaxy A-series phone, both charged at the start: about 28,500mAh in total. Two 20,000mAh power banks would cover it with room to spare.
• A family of four with phones averaging 4,500mAh, all charged at the start: about 60,000mAh in total, or three 20,000mAh power banks.
• Someone living alone with one phone: a single 20,000mAh power bank is plenty for 3 days if the phone starts charged.

Your phone may not have a signal

A charged phone doesn’t guarantee a working network. Be Ready warns that “your mobile phone might stop working if your local mast is in the affected area”, though the network “may still temporarily work in specific regions with backup power”. An Ofcom technical report from February 2025 found power backup at around 20% of UK mast sites for at least one hour, and that only about 5% of sites have backup of at least 6 hours.

A charged phone is still worth having. Signal can come and go, the masts may come back before your power does, and your phone holds your contacts, photos of documents, and offline maps and notes. Emergency Alerts also reach phones that are switched on and in range of a working 4G or 5G mast. And keep a battery or wind-up radio for news.

If you rely on a landline, check how it’s connected. Ofcom says “a digital phone will only work in a power cut if it has a battery back-up”, which providers should offer free to people who depend on their landline. Ofcom’s guidance says it should give at least one hour of access to the emergency services.

Make your battery last

• Turn on low power mode. Apple says Low Power Mode “reduces background activity to extend battery life”. Google says the Pixel 10 can reach “up to 100-hour battery life” with Extreme Battery Saver.
• Don’t use your phone as a torch. Be Ready says it “will drain its batteries more quickly”. Keep a proper torch.
• Keep your phone warm. Apple says iPhones are designed for 0 to 35°C, and that very cold conditions outside that range “may temporarily shorten battery life”. Keep it in a pocket in a cold house.
• Charge up when there’s a warning. When a storm is forecast, charge every phone and power bank to full.

Keep the power bank itself charged

A power bank in a drawer slowly loses charge. Anker says “it’s best to recharge it every three months”. Put a reminder in your calendar, and always top it up before a storm.

Buying safely

• Beware huge, cheap capacity claims. When Australian consumer group CHOICE tested 31 power banks in 2023, “not a single power bank in our test delivered what it promised”. One sold as “500,000mAh” measured just 6,052mAh.
• Buy a known brand from a reputable seller. Electrical Safety First advises buying from “a reputable retailer rather than a third-party seller online”. On Amazon, check that it’s sold by Amazon or by the brand.
• Charge it safely. Electrical Safety First says not to charge battery packs “overnight or while you are out of the house”, and to stop using one that’s swollen, cracked or damaged.
• Avoid extreme temperatures. London Fire Brigade says to avoid storing, using or charging batteries at very high or low temperatures.

Flying with your power bank

Under Civil Aviation Authority rules in force since 27 March 2026, power banks must go in your hand luggage, can’t be recharged on board, and you can take a maximum of two. Power banks up to 100Wh need no approval; 100–160Wh needs the airline’s approval. To work out the Wh, Anker’s formula is mAh x voltage / 1,000. At 3.6V, a 20,000mAh bank is 72Wh.

What about solar chargers and the car?

• Solar is weak in a UK winter. Anker’s solar arm says cloud cover “can cut solar output by 50–90%” compared with a clear sky. It also notes that midwinter days have only 7 to 8 hours of daylight, against 16 to 17 hours in midsummer. Treat solar as a bonus, not your plan.
• Charging from the car works, but never run the engine in a garage. The NHS says “do not leave vehicles or other engines on in enclosed spaces (such as a garage)”, as it can cause carbon monoxide poisoning. Be Ready gives the same warning about using a car radio: in severe weather, “it might be safer to stay inside”.

The power banks worth buying

Frequently asked questions

Sources

• GOV.UK Be Ready — Be ready for emergencies and Power cuts
• Anker UK — How to check the capacity of a power bank and Is a 20,000mAh power bank too much?; Anker — How long does a power bank hold a charge?
• Apple — iPhone 16 specs, iPhone 17 specs, iPhone 17 Pro specs, Low Power Mode and Operating temperatures
• Samsung UK — Galaxy S25 vs S25 Ultra and Best battery life; Google Store — Pixel 10 specs
• Civil Aviation Authority — Changes to rules on power banks
• Ofcom — Mobile network power resilience technical report (PDF) and The future of landline calls; ISPreview — Ofcom guidance on emergency calls in a power cut
• Electrical Safety First — Power banks; London Fire Brigade — Batteries and chargers
• CHOICE — Power banks that don’t live up to claimed capacity
• Anker Solix — Charging a portable power station with solar; NHS — Carbon monoxide poisoning
• Anker — Anker Prime 26K 300W and Anker 737 Power Bank; UGREEN — Nexode 20000mAh 100W

Related reading: how to cool a room during a power cut, how to store emergency drinking water, UK storm names for 2026/27 and our storm-proofing guide.