How long will a power bank or mini UPS keep your router and phone going in a power cut?

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In a power cut, your broadband stops because the router has no power. GOV.UK Be Ready says plainly that your home internet won't work without power (Be Ready). A power bank or a small DC UPS can keep the router going for a few hours, and keep your phone charged. Here's how to work out how long, from the labels you already have.

Will my router and phone work in a power cut?

Be Ready says:

• Home internet won't work without power.
• Mobiles might stop working if your local mast is in the area that has lost power. In some areas the mobile network may keep working for a while on back-up power.
• Emergency calls automatically "roam" onto any available network.
• Corded landlines might still work. For a cordless or digital landline, ask your provider whether it will work in a power cut.

So in a longer cut, both your broadband and your mobile signal could go. Powering the router keeps you online only if the network beyond your home is still working. Read more in does a power cut affect the boiler, hot water and wifi?

How many watts does my router use?

Look at the router's power adapter (the plug or the box on the cable). Find the OUTPUT line. It shows volts (V) and amps (A).

Watts = volts x amps

For example, an adapter that reads 12 V, 1.5 A gives 12 x 1.5 = 18 W.

That is the most the adapter is built to supply. Your router may well use less, so the label figure gives you a cautious answer. If your broadband has a second box that plugs into the mains, it needs power too: add its watts.

Note the volts. Standard USB charging works at 5 V (Anker), but a router adapter may show a different voltage. The battery you use must give the same voltage as the router's adapter, through a matching plug. Check before you buy.

How do I turn mAh into watt-hours?

Power banks are sold in mAh. To compare with watts, you need watt-hours (Wh). Anker gives the formula as mAh = (Wh x 1,000) ÷ V, and says lithium-ion cells are usually 3.6 to 3.7 V (Anker). Turned round:

Wh = mAh x 3.7 ÷ 1,000

If your power bank shows a Wh figure on the case, use that.

Why do I get less than the stated capacity?

Because some energy is lost on the way out. Anker explains that the cells inside work at about 3.7 V, but USB charging needs 5 V, so the power bank's circuits have to step the voltage up. It says a 10,000 mAh power bank has an actual capacity of about 6,000 to 6,500 mAh at 5 V (Anker).

Our working: 10,000 mAh at 3.7 V is 37 Wh stored. 6,000 to 6,500 mAh at 5 V is 30 to 32.5 Wh delivered. That's about 81% to 88%. To keep things simple and cautious, we use 80% below.

For larger portable power stations, Jackery's UK guide uses a similar rule: running time = capacity x 0.85 ÷ watts (Jackery UK).

How long will a power bank run my router?

Hours = power bank Wh x 0.8 ÷ router watts

These use the example router above (12 V, 1.5 A, so 18 W at most). They're examples to show the sum, not claims about any model.

A mini DC UPS for routers works the same way. It sits between the adapter and the router and takes over when the power goes. Use its Wh figure (or convert its mAh), and check it gives the same voltage as the router's adapter.

Our power station runtime calculator does this sum for you.

How many times will it charge my phone?

Charges = usable Wh ÷ your phone battery's Wh

Your phone's battery size is in its specs, sometimes in mAh. Convert it the same way (mAh x the battery voltage ÷ 1,000).

For example, if a phone battery is 15 Wh, a 20,000 mAh power bank (about 59 Wh usable) gives roughly 3 to 4 full charges (59 ÷ 15 = 3.9). Plan on the lower number.

If you need both the router and the phone, they share the same battery. Charge the phone first, then run the router.

More tips in how to charge your phone in a power cut and our guide to power banks in a power cut.

What about my digital landline?

If your landline runs through your router, it stops when the router stops. Be Ready says: contact your landline provider to find out whether your phone will work in a power cut. If it won't, your provider might offer a free solution, such as a back-up battery unit, for a minimum of one hour, so you can call the emergency services (Be Ready). Be Ready's phone and broadband page says vulnerable customers, such as telecare users, can get this free unit (Be Ready).

See digital landlines and power cuts for who qualifies and what to ask.

How do I stay in touch if the mobile network goes down?

Be Ready's phone and broadband page suggests (Be Ready):

• If the mobile network fails but you still have wifi, turn on WiFi Calling. A powered router helps here.
• Try messages: they may work even when calls do not.
• Try public wifi, for example in a café or at a station.
• Move a short distance: signal can sometimes improve.
• Keep a battery or wind-up FM radio for updates.
• Use devices sparingly to save battery.

And before any cut, Be Ready says write down the number to report a power cut: 105 in England, Scotland and Wales, and 03457 643643 in Northern Ireland (Be Ready).

Quick checklist: before the next power cut

• Read the router adapter's OUTPUT line. Write down volts, amps and watts.
• Note the Wh (or mAh) of your power bank or UPS, and check it gives the router's voltage.
• Work out your hours with the sum above.
• Keep power banks charged.
• Ask your landline provider about a back-up battery if you rely on the landline.
• Write down 105 (or 03457 643643 in Northern Ireland).

Limitations

This is general information, not professional advice. Router power use varies by model, and a router may draw less than its adapter label shows. Check your device manuals before connecting anything. In an emergency, call 999.

Related guides

• How many phone charges will a power bank give you in a power cut?
• The digital landline switch: will your phone work in a power cut?
• Power cut kit: what to keep in one box, and how long power cuts last

Free tools

• Power station runtime calculator
• Power cuts near me

We're an independent UK site, not the government. We check guides against official sources and link to them. In an emergency call 999.

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