How to set up router power backup

Last reviewed: 2026-03-04

What this page covers

This is a practical setup guide. It tells you what to buy, how to connect it, and how to test it. If you want to understand the different types of battery backup first, read battery backup basics. If you are not sure which devices in your home need the phone line, start with how to identify what you have.

What you need

There are two common approaches. Pick whichever suits your setup.

Option A: Mini UPS

A mini UPS (uninterruptible power supply) is a small battery unit that plugs in between the wall socket and your router. When the mains power fails, it switches to battery automatically.

You need:

• A mini UPS rated for at least 20Wh (check the product specs). Models designed for routers and networking equipment are widely available online for roughly £25 to £80.
• A spare mains socket near your router

Option B: USB power bank with adapter cable

Some routers can run from a USB power bank using an adapter cable (USB to barrel-jack or USB to the router's power connector). This is cheaper but only works if the router accepts the right voltage.

You need:

• A USB power bank with at least 10,000mAh capacity
• A USB-to-barrel-jack adapter cable that matches your router's power socket and voltage (check the label on the router's original power supply for the voltage, commonly 12V or 9V)

If the voltages do not match, do not use this method. A wrong voltage can damage the router.

What to connect

To keep your phone working in a power cut, these devices all need power:

1. Broadband router (or hub): always. This is the device your digital phone runs through.
2. ONT (fibre box): only if you have full-fibre (FTTP). This is the small white box where the fibre cable enters your home. It has its own power supply. If the ONT is off, nothing works, even if the router has power.
3. Phone adaptor: only if your provider uses a separate box between the router and the phone handset. Not all setups have one.

You do not need to connect your phone handset if it is a corded phone. Cordless phone base stations need mains power, but many have a short internal battery that lasts 30 minutes to a few hours.

Step-by-step setup

Mini UPS setup

1. Plug the mini UPS into the wall socket. Use the same socket that currently powers the router (or a nearby one on the same circuit).
2. Let the UPS charge fully. Most units take 4 to 8 hours to reach full charge. The indicator light or display will show when it is ready. Do not rely on it until it is fully charged.
3. Plug the router's power cable into the UPS instead of directly into the wall socket.
4. If you have an ONT (fibre box), plug its power cable into the UPS too. Use a second output socket on the UPS if available. If the UPS only has one output, you may need a small extension lead between the UPS and the devices.
5. If you have a phone adaptor, plug that in as well.
6. Check everything is working normally. The router should be on with its usual lights. Pick up the phone and listen for a dial tone.

USB power bank setup

1. Charge the power bank fully.
2. Connect the adapter cable from the power bank's USB output to the router's power socket. Do not disconnect the router's normal power supply yet.
3. Test the connection (see next section) before relying on it.

The power bank method does not switch over automatically. In a power cut, you will need to unplug the router's mains power cable and plug in the power bank manually. This means a short gap with no service while you switch over.

This method also does not cover the ONT. If you have full-fibre (FTTP), the ONT needs its own power source, and most ONTs cannot run from USB. A mini UPS is the better choice for FTTP connections.

How to test it

Do this as soon as you have set everything up.

1. Turn off the mains power at the wall socket (or switch off the circuit at the consumer unit).
2. Watch the router. It should stay on. The UPS should show it is running on battery. If the router turns off and reboots, wait for it to come back up (this can take 1 to 3 minutes).
3. Pick up the phone and check for a dial tone.
4. Make a test call to a mobile or another landline. Confirm the call connects and both sides can hear each other.
5. Turn the mains power back on. The UPS should switch back to mains and begin recharging.

If the router does not stay on, check that all cables are seated properly and that the UPS is charged. If it still fails, the UPS may not provide enough power for your router model. Check the wattage ratings.

How often to re-test

Test the battery backup every 3 months. Batteries lose capacity over time, and a unit that worked six months ago may no longer hold enough charge. Set a reminder on your phone or write the next test date on a sticker on the UPS.

Replace the UPS battery (or the whole unit, for sealed models) when the runtime drops below what you need. Most mini UPS batteries last 2 to 3 years with normal use.

Runtime estimates

How long the backup lasts depends on the battery size and what you have connected. These are rough figures:

• Small mini UPS (20-30Wh), router only: 2 to 4 hours
• Larger mini UPS (50-100Wh), router only: 4 to 8 hours
• Router plus ONT (fibre box): reduce the above by roughly a third
• USB power bank (10,000mAh), router only: 1 to 3 hours (varies widely by router model)

Under Ofcom's rules, providers must offer at least one hour of emergency-calling backup for customers who depend on the landline. A provider-supplied unit will typically meet that minimum. If you need longer coverage, a larger mini UPS is the simplest option.

Common questions

Can I use a large UPS meant for a computer?

Yes, it will work. But a desktop UPS is heavier, bulkier, and more expensive than needed for just a router. A mini UPS designed for networking equipment is simpler and takes up less space.

What if my router reboots when power switches to battery?

Some routers briefly lose power during the switchover, especially with cheaper UPS units. The router will restart on its own, but this takes 1 to 3 minutes. During that time, you cannot make calls. A UPS with a faster switchover time (listed in the specs as "transfer time") reduces this gap. Look for units with a transfer time under 10 milliseconds.

Should I leave the UPS plugged in all the time?

Yes. The UPS is designed to stay plugged in and charged. It only uses battery power when the mains fails. Leaving it connected keeps it ready.