A charged phone is your lifeline in a power cut — for checking the outage map, calling the network's fault line, keeping in touch with family or getting emergency alerts. A power bank is the cheapest, simplest way to keep it running when the mains is off, and it earns its place in a home kit, a car and a grab bag alike. As UK winters bring more storm-driven outages, it has quietly become everyday household kit rather than travel gear. This guide covers what actually matters when you buy one, and where the numbers on the box are worth ignoring.
How much capacity do you actually need?
Capacity is quoted in milliamp-hours (mAh), and bigger is not automatically better — it's a trade-off against size, weight and charging time. A 10,000mAh bank is pocket-sized and gives most phones roughly one and a half to two full charges: plenty to keep one phone alive through a day-long outage. A 20,000mAh bank is about the size of a paperback and covers two phones over a couple of days, or a phone plus a tablet or head torch. For most UK households, a 20,000mAh bank as the main one and a small 10,000mAh for pockets and the car is a sensible pairing. Going bigger than 20,000mAh moves you towards a portable power station — a different, heavier tool for running lamps and small appliances.
Why you never get the full mAh out of it
This trips people up, so it's worth knowing. A 10,000mAh power bank does not put 10,000mAh into your phone. The bank's cells are 3.7 volts while your phone charges at around 5 volts, and converting between them loses energy as heat — so real-world output is roughly 60 to 70 per cent of the label. A 10,000mAh bank therefore delivers around 6,000 to 7,000mAh of usable charge, which is still one and a half to two charges for a typical phone. When you compare banks, treat mAh as a rough guide, not a promise, and don't feel short-changed when the maths doesn't line up — that's physics, not a faulty unit.
USB-C, ports and cables
Buy USB-C. It's now the standard across phones, tablets, head torches and most kit, and from late 2024 new phones and small electronics sold in the UK use USB-C by law, so a USB-C bank future-proofs your kit. Look for a bank that can both charge your devices and recharge itself over USB-C, ideally at speed. Two output ports let you charge two phones at once, which matters in a household. A few banks now have a built-in or retractable cable, so there's one less thing to lose in a drawer — genuinely handy for a grab bag — though a bank with standard ports plus a spare cable taped to it works just as well.
Fast charging — useful, but read it right
Charging speed is measured in watts (W). A basic bank puts out around 10 to 12W, which fills a phone in a couple of hours. A fast bank rated 20W or more can take many phones from flat to half in half an hour, which is worth having when you're grabbing a top-up between other jobs during an outage. The catch is that the phone, the bank and the cable all have to support the same fast standard, or you drop back to slow charging — so a high wattage on the bank alone guarantees nothing. For charging laptops you need 45W or more; most people don't, and a lower-wattage bank is smaller, cheaper and perfectly good for phones and torches.
Pass-through and recharging the bank
Think about how you'll refill the bank itself. At home you'll top it from the mains, but in a longer outage that's the whole problem. A bank that recharges over USB-C can be refilled from a car USB socket, a laptop or a folding solar panel, which keeps you going off-grid. 'Pass-through charging' means the bank can charge a device while it is itself being charged — useful but not essential. More important is simply keeping the bank topped up: a power bank that's flat in the drawer is no use at all when the lights go out.
Cold, safety and air travel
Lithium cells lose capacity in the cold, so a bank left in a freezing car will underperform until it warms up — keep the one you rely on indoors and bring the car one inside overnight in winter. Buy from a recognised brand: cheap unbranded banks are where the horror stories about swelling and overheating come from, and a reputable one with over-charge and short-circuit protection is worth the few extra pounds. One practical note for grab bags: airlines require power banks in carry-on, not checked luggage, and usually cap them around 100Wh (roughly 27,000mAh), so a 10,000 or 20,000mAh bank is fine to fly with.
A sensible plan for most UK homes
For most households the best setup is one 20,000mAh USB-C bank from a recognised brand as the main household charger, kept topped up and stored with a charging cable, plus a small 10,000mAh bank for pockets, the car and the grab bag. Aim for 20W or faster output if you want quick top-ups, add a folding solar charger if you want a way to refill off-grid in a longer outage, and pair the lot with a USB head torch and radio so one charged bank keeps your essentials running. Charge everything when the clocks change, check the levels a couple of times over winter, and you'll never watch your phone die during a storm.
