The short answer
Your mobile will work in a power cut for as long as its own battery lasts and a mast nearby still has power. Broadband and a digital landline will not work at all unless you have battery backup on the router, and on a full fibre line also on the fibre box. Only an old copper landline with a corded phone keeps working on its own.
If you depend on your landline and have no working mobile or reliable signal, Ofcom says your provider must offer you a free battery backup that lasts at least one hour. In a real emergency, call 999 or 112 first. This page is reference material, not professional advice.
What happens by technology
| What you use | In a power cut at your home | What the rules and operators say |
|---|---|---|
| Mobile phone | Works on its own battery. Calls need a mast with power, and most masts have little backup. | Ofcom: around 20% of UK mast sites have power backup of at least one hour, and far fewer (5%) have 6 hours or more. Your phone can use another network to call 999. |
| Copper landline, corded phone | Keeps working. | Ofcom: the lines are powered from the local telephone exchange. |
| Copper landline, cordless phone | Usually stops. The base unit needs mains power. | Ofcom: cordless phones are very unlikely to work and tend to have no battery. |
| Digital landline (over broadband) | Stops, unless the router has battery backup. | Ofcom: digital landlines need mains electricity. Providers must offer a free solution to people who depend on their landline. |
| Full fibre broadband | Stops, because the fibre box (ONT) has no power. | Openreach: unplug the ONT and broadband stops at once. BT: the backup unit has to power the Openreach modem too. |
The rows overlap in practice. A digital landline is a phone service that runs on your broadband, so a house with full fibre and a digital landline needs both the router and the fibre box on battery.
Mobile: your phone is fine, the mast is the question
A charged phone is the most useful thing in a power cut, and Ofcom agrees that for most people a mobile is the practical alternative if the power goes out. The weak point is the network around you, not the handset.
In a technical report published on 10 February 2025, Ofcom estimated that around 20% of all mast sites in the UK have power backup of at least one hour. It found that far fewer, 5%, have backup of at least 6 hours. Ofcom also quotes Ofgem data saying most power cuts, approximately 90%, last between 3 minutes and 3 hours. So a short cut is usually survivable on mobile, while a long one is where coverage starts to disappear.
The same report points out that this is a risk and not a certainty. During Storm Arwen, it says, over 74,000 customers lost mains electricity for over 48 hours, and mobile service was disrupted as thousands of cell sites lost power too. That is the kind of event where the mast list above matters.
Two things help. If your own network is down, calling 999 will connect through another mobile network if one is available. If your phone shows signal but calls will not connect, Ofcom suggests sending a standard text message instead. Ofcom also advises keeping a battery power bank, since it can recharge your devices if the severe weather lasts a long time.
One more trap: calls over Wi-Fi do not help when the router is dead. Ofcom notes that messaging services such as WhatsApp work over your broadband connection, so they stop with it unless you have backup.
Copper landline: the old line is the exception
Ofcom's guidance says traditional corded telephones on the old copper network can still be used to make calls in a power cut, because the lines are powered from the local telephone exchange. That is why people remember the phone "always working".
Two catches. A cordless handset needs its base station plugged into the mains. Ofcom says these do not tend to have a battery. And the copper network is being retired in favour of digital landlines, so your line may already have been moved. Ofcom's page on the switchover tells you to speak to your provider if you are not sure.
Digital landline and broadband: no mains, no service
A digital landline works over a broadband connection, and Ofcom says you plug your phone into your broadband router rather than the wall socket. The router needs mains electricity. Ofcom puts it plainly: digital landlines will not work in a power cut unless there is a battery backup. BT says the same of its Digital Voice service, whose hub needs power to make calls.
The backup rules for people who depend on the phone come down to three points.
- Ofcom's guidance says providers should have at least one solution that gives access to emergency organisations for a minimum of one hour in a power cut at the home.
- If you do not have a working mobile phone or a reliable mobile signal, your provider must offer a battery backup for free. Ofcom says it must last at least one hour, and some providers go up to 7 hours.
- BT recommends its backup unit if you use a health alarm or medical pendant on Digital Voice, or have no mobile as an alternative.
Note what the rule covers. It is about reaching emergency organisations, not about keeping your whole home on the internet for a day. Some providers offer more: Vodafone describes a small broadband battery unit that keeps users connected to WiFi for up to 4 to 7 hours, depending on the router. BT says its unit keeps the hub and Digital Voice working for at least an hour.
Full fibre: the fibre box needs its own backup
With full fibre, the connection point in your home is an Openreach box called the ONT, which has a small power supply. Openreach says that if you unplug it, your broadband stops working immediately, including Wi-Fi and wired connections. A power cut does the same thing.
That is why BT's instructions say that with Full Fibre Broadband the backup unit must also be connected to the Openreach modem. A backup that powers only the router leaves you with a router and no connection. Ask your provider which unit it supplies, and whether it covers the fibre box.
Extra help from your electricity network
Phone backup from your landline provider and the electricity register are two separate things. Ofgem says network operators must offer customers on the Priority Services Register extra help, for example battery packs so they can still charge phones or medical equipment. That is about the electricity side. It does not replace the landline backup that Ofcom requires, so if you rely on the phone line, ask your phone or broadband provider as well.
For the rest of a power cut, from food to heating, see our guide on what to do in a UK power cut. We are also building Waystone, an offline preparedness assistant that is still in development and not for sale. It runs on a laptop with no connection, which is the same reason we care about backup power.
Common questions
Will my mobile work in a power cut?
Your phone will work on its battery, but calls depend on a nearby mast that still has power. Ofcom estimates around 20% of UK mast sites have backup of at least one hour. If your own network fails, 999 calls can connect through another network.
Will my landline work in a power cut?
A copper landline with a corded phone usually does, because the exchange powers the line, according to Ofcom. A cordless phone, or any digital landline plugged into a router, will not work unless there is a battery backup.
Will Wi-Fi or broadband work in a power cut?
No, unless the router has battery backup. On full fibre the Openreach box also needs power, so BT's instructions have its backup unit connected to the modem as well as the hub.
Can I get a free battery backup?
Ofcom says that if you do not have a working mobile phone or a reliable mobile signal, your landline provider must offer a free backup such as a battery unit. It must last at least one hour. Contact your provider and tell them you rely on the landline.
How long does a router battery backup last?
Ofcom's minimum is at least one hour, and it says some providers have backups that last up to 7 hours. BT's unit keeps its hub going for at least an hour, and Vodafone describes up to 4 to 7 hours depending on your router.