Power and solar backup basics: sizing a setup that actually works

Search for "backup power" and most of what comes up is trying to sell you the biggest box with the most sockets. There's a better question to ask first: what would I actually need to keep running, and for how long? This guide answers that with some simple arithmetic before you spend anything. For what a long outage looks like hour by hour, start with what if the power went off for 48 hours?.

Work out your own watts and watt-hours first. For a lot of households the honest answer is a torch, a radio and a power bank, with no battery box at all. A power station (a big battery with sockets on it) earns its place for specific reasons: someone's medical equipment, a home office you can't afford to lose, or wanting the fridge running the whole time. Plug-in solar, however handy on ordinary days, won't do any of that.

Before you buy anything

Two things matter more than any kit. First, 105 is the free number to report a power cut, and it puts you through to your network operator rather than the company that sends your bill. Second, if anyone at home relies on electricity for medical or mobility equipment, sign up to the Priority Services Register, a free service that tells your network operator you need extra help, and talk to your care team about a plan. The Met Office and GOV.UK's Prepare site both say the same.

Then think about how long outages last. Most are short, but not all. During Storm Arwen in November 2021, Ofgem found that while 83 per cent of the roughly one million customers affected were back on within 24 hours, nearly 4,000 were off for more than a week. So plan for a few hours, and make sure the plan can stretch to two days without much extra effort.

The options, honestly

• A torch, a radio and a power bank are cheap and cover phones, light and news. Most households don't need a battery box for the first day or so.
• A portable power station is a battery with sockets. It's silent, gives off no fumes, is safe indoors, and recharges from the mains or a solar panel. It's limited by its capacity: it'll run small things for a long time or bigger things for a short time. It's worth having if you need power for medical equipment, a work setup, or a fridge that runs throughout.
• A petrol or gas generator gives you far more power for your money, but it needs fuel and a plan for when that runs out, and it must only ever be run outdoors. The NHS says not to run engines in enclosed spaces such as garages because of carbon monoxide, which you can't see or smell. Keep it well away from windows, doors and air bricks, and fit a carbon monoxide alarm; the UK Health Security Agency calls one the single most important thing you can do.
• Rooftop solar with a battery is a different decision altogether. It's a long-term investment that lowers your bills, with backup in a power cut as a possible bonus, and it isn't the same thing as plug-in solar (more below).

Plug-in solar is not backup power. The kits that became legal in Britain in August 2026 are designed to switch off when the grid does, and they can't include a battery. They cut bills on ordinary days, and our plug-in solar guide covers what they save and the rules. Ordinary rooftop solar without a battery also switches off in a power cut, so if backup is part of the reason you're buying, ask the installer directly.

The sums: watts and watt-hours

Two different numbers matter here, and mixing them up is the most common mistake.

• Watts (W) is how much power something draws at any moment. It decides whether a power station can run it at all. Anything with a motor, like a fridge compressor, draws extra for a moment when it starts up, so check the maker's surge rating.
• Watt-hours (Wh) is watts multiplied by hours. It decides how long the battery lasts, and a power station's capacity is given in Wh.

Typical figures vary a lot between models, so check the label on yours. Roughly: a router and modem use about 10 to 20 W; charging a phone or laptop takes a few tens of watts; an LED bulb is 5 to 10 W; a CPAP machine is about 30 to 60 W (check yours); and a fridge-freezer draws a hundred watts or so while its compressor runs. An electric heater pulls 1,000 to 3,000 W, which is why it's the first thing to leave off the list.

A worked example, using your energy label

The most useful number on a fridge isn't its wattage but its energy label, which gives kilowatt-hours a year. Divide by 365 for the daily figure. Here's an example 24 hours, with our own assumptions marked:

• A fridge-freezer labelled 250 kWh a year uses about 685 Wh a day.
• A router at 15 W all day uses 360 Wh.
• Phones: two full charges each for two phones, at roughly 15 Wh a charge, comes to about 60 Wh.
• Lights: three LED bulbs at 8 W for five hours use 120 Wh.

That's about 1,225 Wh a day. A 1,000 Wh power station wouldn't quite manage all of it, and some energy is always lost in the conversion, so check the maker's usable figure. Two things jump out. The router is 30 per cent of the total, and as our internet-outage guide explains, it may have nothing to connect to in a widespread cut. Leave it out and the day drops to about 865 Wh. And the fridge is over half of it, yet the Food Standards Agency's figures show that a closed fridge and a full freezer already get you through the first day if you eat in the right order (see the 48-hour guide).

Can solar recharge it?

A folding panel can top up a power station, but a British autumn or winter is exactly when you'd need it and when it gives the least. For a rough idea, here's our own arithmetic on the Energy Saving Trust's estimate that an 800 W plug-in kit facing south, unshaded and tilted, makes about 700 kWh a year. That's roughly 875 kWh per kilowatt of panel per year. A 200 W panel would make about 175 kWh a year, or about half a kilowatt-hour a day on average across the year, and much less in winter or if it isn't pointed well. So a single portable panel helps over days rather than hours, and for a 48-hour winter outage you should plan around the battery alone.

Whole-home solar and battery

This is a different purchase from backup power. The Energy Saving Trust puts the average cost of a rooftop solar system at around £7,600. If you're thinking about one, get at least two quotes. Ask specifically whether it can keep your home running in a power cut (many grid-tied systems can't, because they're designed to shut down when the grid fails) and how big the battery is, and check which circuits it would keep going, since many only back up selected ones. We don't quote a payback period, because it depends entirely on your usage, your roof and prices at the time.

A realistic first build, in order

1. Write down 105 and your key numbers on paper, and look at the Priority Services Register if it applies to you.
2. Get the cheap kit first: a torch each, a battery or wind-up radio, and a charged power bank.
3. Do the sums for what you'd really want to keep running, using the energy-label method.
4. Decide whether you need a battery at all. If you do, choose by watt-hours for that list, with some headroom for start-up surges, not by the biggest number on the box.
5. Only then think about solar to recharge it, bearing in mind how little a single panel adds in winter. A generator is a separate choice with its own safety rules.

The two-futures view

If the grid stays reliable, a small power bank, a torch and a few minutes of arithmetic cost you almost nothing. If it doesn't, the same arithmetic tells you exactly what a bigger setup would need to do, before a bad week forces your hand. Either way the sums are free, and they stop you paying for capacity you don't need.

Frequently asked questions

Where to go next

• What if the power went off for 48 hours?: what actually stops, hour by hour, and what to eat first.
• Plug-in solar is now legal in Britain. Is it worth it for your home?: what it saves on ordinary days.
• How long could you feed your household?: the same planning for food and water.
• What if the internet went down for seven days?: why a digital landline stops working in a power cut.