Work out which boxes keep you online

Before buying anything, follow the broadband from where it enters your home to the device you actually use, and note every box with a power lead. That short chain is all you need to keep alive. Everything else in the house can stay dark.

What sits on that chain depends on your broadband type:

• Full fibre (FTTP): a fibre cable comes into a small box on the wall called an ONT (Optical Network Termination), and that feeds your router. Both need power. This is the setup people most often get wrong, because the router can stay lit and broadcasting WiFi while the unpowered ONT quietly kills the actual connection.
• Fibre to the cabinet (FTTC) and cable: usually just the one router, or a modem and router, to power. The green street cabinet down the road has its own battery, typically good for a few hours.
• Mesh WiFi: only the main node that plugs into the router has to stay up for basic internet. The satellite nodes can sit in the dark unless your desk only gets a signal from one of them.

A quick way to check for an ONT: look where the phone or fibre line enters, usually near the front door or in the hall, and see if there is a second small box with lights beside the router. If there is, assume it needs power too, and plan to put both on the same backup.

One honest expectation to set now: keeping these boxes powered protects you against short, local cuts, which are the common kind. It does nothing for the internet arriving at your door in the first place, and we come back to that limit at the end.

Keep the router and ONT powered with a mini UPS

The neatest answer for most homes is a mini UPS: a small battery pack that sits between the mains and your router, passes power straight through while the electricity is on, and switches to its own battery the instant the mains drops. Because the changeover happens in a few milliseconds, the router never reboots and your call or download carries on as if nothing happened.

A mini UPS made specifically for broadband routers is the tidiest version of this. It outputs the low voltage the router expects, so there are no adaptors to puzzle over, and it is small enough to tuck behind the kit. Put the router on it, and if you are on full fibre, run the ONT from it too so both boxes ride through the cut together.

Sizing is reassuringly simple because the load is tiny. A router draws roughly 8 to 15W and an ONT another 5 to 10W, so budget about 15 to 25W for the pair. As a rough runtime guide, take the usable watt hours, knock off about 20 per cent for conversion losses, and divide by the load in watts to get the hours. On that maths a 300Wh battery at 20W keeps you online for most of a day, while a small mini UPS is aimed at bridging the short cuts and brownouts that make up the vast majority of UK outages.

What to look for

• An output that matches your router's voltage, almost always 12V DC. There is more on matching this safely in the next step.
• Enough outputs, or a spare barrel cable, to run the ONT as well as the router.
• Pass through charging, so the unit stays topped up and only discharges when the mains actually fails.
• Enough capacity for the runtime you really want. An hour to ride out short cuts is plenty for most homes; aim higher if outages near you tend to be long.

One habit worth keeping: do not leave the router running off a small battery that never fully recharges, and test the unit once by pulling the mains plug so you know it holds before you need it in the dark.

Match the voltage and connector before you buy

Whether you use a mini UPS, a portable power station or the power bank trick in the next step, the one thing that can go wrong is voltage. Get it wrong and you can permanently damage the router, so spend two minutes checking before you plug anything in.

Look on the underside of the router, or on its mains adaptor, for the small print showing the voltage and current it needs. Most UK consumer routers want 12V DC through a round barrel connector measuring 5.5mm across the outside and 2.1mm in the middle (often written as DC5521), drawing somewhere between 0.5 and 3 amps. A few rules follow from that:

• The voltage must match exactly. A higher voltage can fry a router that has no over voltage protection. Just because a plug physically fits does not mean it is safe.
• More amps are fine. A battery that can supply more current than the router needs is not a problem, because the router only draws what it wants. It is the volts you must match, not the amps.
• Check the polarity. Barrel connectors can be centre positive or centre negative. UK router supplies are almost always centre positive, but check the little symbol on the adaptor and make sure any cable you buy matches.

If you are feeding the router from a USB battery rather than a mains output UPS, the USB port type decides what you need. A USB-A port is fixed at 5V, so it needs a 5V to 12V step up converter cable, and that only suits lower power routers. A USB-C port that supports Power Delivery can serve 12V directly with the right trigger cable or adaptor, which is the more reliable option. Just check the adaptor is set for 12V and not the 18V or 20V that some default to.

Two exceptions worth flagging. BT Smart Hubs use a different, less common barrel size (believed to be 5.5mm by 3.0mm) that is hard to source in the UK, so a UPS with a normal mains (AC) output is often the easier route for a BT hub. And if any of your kit only takes a three pin mains plug, a mains output UPS or a portable power station sidesteps the whole voltage matching question.

Have a power bank and mobile fallback ready

Even with the router backed up, it is worth having a second line of defence, both for longer cuts and for the times the fixed line goes down completely.

The power bank trick

Many 12V routers will run straight off a USB power bank, the same sort you use to charge a phone, if you add the right cable. A USB-C Power Delivery bank with a USB-C to 12V trigger cable, or a USB-A bank with a 5V to 12V step up converter, can keep a low power router going for hours. A charged power bank earns its place in a power cut two ways: it keeps your phone alive so you can use mobile data, and with the right lead it can feed the router itself. A compact one, like the kind you already carry for travel, is enough to get started. Keep it charged and stored with the correct cable so you are not hunting for adaptors by torchlight.

Falling back to mobile

If the fixed broadband is down but your phone still has a signal, tethering or a mobile hotspot is the quickest way back online. Mobile masts have their own battery and generator backup and often stay live longer than fixed line kit in a local outage, which is why your phone frequently keeps working when the broadband does not.

Know its limits, though. In a widespread or long cut, masts get busy as everyone reaches for the same fallback, and their backup batteries do not last forever, so speeds can crawl or drop out. Treat mobile as the plan that keeps a laptop and a phone going for the essentials, not as a full replacement for home broadband. If you rely on it, check you actually get a usable signal indoors before you need it, and keep that power bank charged so the phone doing the tethering does not die first.

Know what your battery cannot fix

This is the part most guides skip, and it is the most important. Backing up your own kit only helps if the problem stops at your front door. Plenty of it does not.

The line outside can still be down

Your battery keeps your router and ONT alive, but it cannot power the fibre distribution point in your street, the green cabinet, or the exchange further up. If the cut is wide enough to take those out, your broadband goes down the instant the power does, backup or not. UK full fibre users have reported exactly this: careful home battery setups that kept every light on indoors, and the connection still dropped the moment the area lost power, because the network outside had no backup of its own. It is still worth backing up for the short, local cuts, which are far more common. Just do not expect it to defeat a regional outage.

Digital Voice landlines need the broadband up

The old analogue phone line that carried on working in a power cut is being switched off. Most home phones now run over the broadband as Digital Voice or a similar VoIP service, which means the phone only works while the router, the ONT and the handset base are all powered and the broadband itself is up. Providers offer a battery backup unit that keeps the phone going for around an hour, and they will prioritise vulnerable customers, but if the line outside is down no amount of home backup will get you a dial tone. If someone in the house depends on the phone for care or emergencies, ask your provider what they offer and do not assume the landline is independent of the mains any more.

So what should you actually buy

• Most homes: a mini UPS sized to hold the router, plus the ONT on full fibre, through the short cuts. Match the 12V and you are done.
• Want more than WiFi: a portable power station instead, so it also runs phone chargers, a laptop and a lamp. More useful, but pricier and bulkier.
• On a budget, or already own a power bank: the USB power bank plus the correct step up or PD cable, kept charged and ready.
• Everyone: a charged phone with a tested hotspot as the fallback, and, if you have a Digital Voice landline, a vulnerable user plan with your provider.

Whatever you choose, test it once with the mains switched off while it is light and calm, so the first time it matters is not the first time you find out whether it works.