How Much Backup Power Do You Actually Need in a UK Power Cut?

Most winter power cuts in the UK are short — a couple of hours while the network company reroutes supply. But storms like the ones that took out power to hundreds of thousands of homes in recent winters can leave rural and exposed properties off-grid for a day or more. The honest question isn't "what's the biggest battery I can buy" — it's "what do I actually need to keep running, and for how long?" This guide walks through that in plain watt-hours, so you buy the right size once instead of over-spending or being caught short.

The one number that matters: watt-hours (Wh)

Ignore the marketing headline of "20,000mAh" or "huge capacity" for a moment. The figure that lets you compare a phone power bank to a fridge-sized power station is the watt-hour (Wh). It tells you how much energy is stored. To estimate how long something runs: divide the battery's watt-hours by the device's power draw in watts. A 240Wh power station running a 20W LED lamp lasts roughly 240 ÷ 20 = 12 hours (allow ~15% for conversion losses, so call it 10).

To convert a power bank's mAh to watt-hours: multiply the mAh by the battery voltage (usually 3.7V for the cells), then divide by 1,000. A 20,000mAh bank is about 20,000 × 3.7 ÷ 1,000 = 74Wh. That's why a "big" 20,000mAh power bank and a "small" 240Wh power station are in completely different leagues — the power station holds roughly three times the energy.

What each tier of backup power realistically runs

A power bank (roughly 37-74Wh)

This is the backbone of any home kit and the first thing everyone should own. A 10,000mAh bank (about 37Wh) recharges a typical phone one and a half to two times; a 20,000mAh bank (about 74Wh) does three to four phones or keeps one phone, a head torch and a small radio topped up through a short outage. It cannot run anything mains-powered — no kettle, no fridge, no router with a mains plug. But for the most common two-to-six-hour UK cut, keeping phones alive for 999/105/family contact is usually all you actually need.

A portable power station (roughly 240-1,000Wh)

This is a proper mains-plus-USB battery with real sockets. An entry 240Wh unit will run a laptop for several charges, keep a router and phones going, power a CPAP machine or nebuliser overnight (check yours — many run 10-30W), or run a 15-20W LED lamp for the best part of a day. Step up to 500-1,000Wh and you can keep a fridge-freezer ticking over in short bursts, run a small heated blanket, or power medical devices for longer. What it won't do cheaply is run high-heat appliances — kettles, toasters, fan heaters and hair dryers pull 1,500-3,000W and will flatten even a large station in minutes, so plan to boil water on a gas or off-grid stove, not on mains.

A solar panel (recharge, not storage)

A folding solar panel doesn't store anything — it refills a power bank or power station during daylight, which is what turns a one-day kit into a multi-day one. Be realistic about British winter light: a 100W panel might deliver 30-60Wh an hour in bright winter sun and far less under thick cloud, so treat solar as a top-up that extends your battery, not a replacement for it. It earns its place if you live somewhere prone to long rural outages.

Work out your own number in five minutes

List the few things you genuinely must keep running, note each one's wattage (it's usually printed on the device or charger), and multiply by the hours you want them to last. Add them up and that's your target watt-hours.

Worked example — a typical family, aiming for one day off-grid: two phones topped up twice (~30Wh), a broadband router at 10W for 12 hours (120Wh), an LED lantern at 5W for 8 hours (40Wh), and a laptop charged once (~60Wh). That totals about 250Wh, which an entry-level ~240-300Wh power station covers comfortably with a 20,000mAh power bank as backup for the phones.

Worked example — someone reliant on a medical device: check the device's wattage first (many CPAP machines run 30-60W with humidifier off). For 8 hours at 40W you need about 320Wh usable, so a 500Wh station gives you genuine overnight margin plus phone charging. If your health depends on power, sign up for your network operator's free Priority Services Register and keep the station charged and ready — don't rely on solar to top it up in an emergency.

A sensible layered kit for most UK homes

For the majority of households the best-value approach is layered, not a single big purchase. Own a good 20,000mAh power bank and keep it charged — it handles the short cuts that make up most outages. Add an entry portable power station (around 240-300Wh) if you have a medical need, work from home, or live somewhere storms cut you off for longer. Add a folding solar panel only if long rural outages are a real risk for you. Keep torches and a lantern on lithium AA or their own built-in cells so your lighting never competes with the battery you're rationing for phones and medical kit.

Buy once, size it right

The trap is buying by headline capacity or price rather than by what you need to run. Do the five-minute watt-hour sum above, size to a comfortable margin over that, and keep whatever you buy charged and accessible — a flat power station in the cupboard is worth nothing at 2am in a storm. For most people a charged power bank covers 90% of real UK power cuts; a modest power station and a torch collection cover almost all of the rest.