Keeping communications online is often the first priority during a power cut. A broadband connection can help you receive updates, contact family, work remotely and make calls over the internet, yet the equipment usually draws far less power than a kettle, heater or fridge.
So, how much backup power does a broadband router need? Usually, the router itself is a low-wattage load. The harder part is running it for long enough, allowing for its power adapter, any fibre equipment, and the phones or laptops you want to use alongside it.
Why router backup is mainly a runtime question
A router may consume only a modest amount of electricity while operating, but an outage can last for several hours. That makes energy capacity more significant than headline inverter wattage. A small backup system may be able to supply the router’s running load comfortably, but still run out of stored energy sooner than expected if its battery is small or other devices are connected.
There is also more than one box to consider. A typical UK broadband setup might include:
- the broadband router or hub;
- an optical network terminal (ONT) for a fibre-to-the-premises connection;
- a separate modem, if your service uses one;
- mesh Wi-Fi nodes, switches or other network equipment; and
- a landline handset or cordless phone base that depends on mains power.
ADSL, fibre-to-the-cabinet and full-fibre installations do not all use the same equipment. Check what is actually plugged into the wall rather than sizing the backup around the router name alone.
Find the router’s real wattage
Look first at the router’s mains power adapter. It may show an output such as volts and amps rather than a watt figure. The rough electrical relationship is:
Watts = volts × amps
That calculation describes the adapter’s rated output, not necessarily the router’s continuous consumption. An adapter can be rated above the power the equipment normally uses, and actual demand varies with model, Wi-Fi activity and connected hardware. If the manufacturer publishes typical or maximum input power, use that information in preference to an assumption.
A plug-in power meter can provide a useful indication of the combined mains draw, provided it is used safely and its reading is treated as an observation under those particular conditions. Measure the router and any separate ONT together if they will share the same backup supply.
Do not confuse the router’s low running load with its adapter’s label, and do not assume every device with a similar appearance uses the same voltage or polarity. The replacement or backup supply must be electrically compatible. If you are unsure about a DC connection, use the manufacturer’s guidance or ask a suitably qualified technician.
How to calculate router runtime
For a battery-based system, begin with energy rather than watts. Battery capacity is commonly expressed in watt-hours (Wh), while the router’s demand is measured in watts (W).
Approximate runtime in hours = usable battery energy in Wh ÷ total load in W
For example, a system with 100 Wh of usable energy supplying a combined 10 W load has a simple theoretical runtime of 10 hours. Real runtime will be lower or otherwise different if the battery cannot deliver its full nominal capacity at that load, if the inverter or DC converter loses energy, or if the system reserves capacity to protect the battery.
Nominal battery capacity is not the same as usable energy. Manufacturers may state a battery’s total rated capacity, while the available output depends on battery chemistry, permitted depth of discharge, temperature, age and the backup unit’s control system. Inverter efficiency matters too. A conventional mains-output UPS may consume some of its own energy while converting battery power to 230V AC, whereas a compatible DC backup arrangement can avoid some conversion steps.
For planning, use a margin rather than selecting a battery that only meets the ideal calculation. Compare the manufacturer’s stated runtime at a similar load where available, and treat it as model-specific rather than a universal promise.
Choosing a UPS or battery backup for broadband
A small UPS is often convenient because the router and its accessories remain connected to normal mains power while the UPS handles a cutover. Check three separate things:
- Output power: the UPS must support the combined running load, with suitable allowance for any brief starting or inrush demand.
- Energy capacity: this determines how long the equipment can operate during the outage.
- Connection compatibility: the output socket, voltage, frequency and any DC leads must suit the equipment. UK mains equipment is designed around 230V and 50Hz, but the exact requirements belong to each device.
Router power supplies are generally not large motor loads, so starting surge is often less troublesome than it is with pumps, compressors or refrigeration. It should still be checked if you are adding a larger device or relying on a particular inverter’s limits. A UPS with a waveform or output specification that is unsuitable for the connected power supply can cause nuisance behaviour, so follow the UPS and equipment manufacturers’ compatibility guidance.
Some dedicated networking battery units provide a selection of low-voltage DC outputs. They can be efficient, but only use an output that matches the router’s required voltage, connector and polarity. A connector that fits physically is not proof of electrical compatibility.
The [UK Backup Power Calculator](https://backuppowercalculator.co.uk/) can help you turn your appliance list into a more realistic wattage, surge and runtime requirement. Include the router, ONT and every other device that you expect the backup system to supply.
Allow for phones, laptops and other communications equipment
Keeping the router powered does not keep every communication device charged. Phones may continue using their internal batteries, but laptops, tablets and cordless phone bases add their own energy requirements. A laptop can also change its demand substantially depending on screen brightness, processor activity, charging state and whether it is running from its battery.
Make a short outage plan:
- List the router and any ONT or modem that must remain on.
- Decide how many hours of internet access you actually need.
- Identify whether a phone, laptop or tablet must be charged during that period.
- Measure or check the input rating for each item rather than guessing from its size.
- Add the loads together before calculating the required battery energy.
For instance, a router and fibre box might form a small continuous load, while briefly charging a laptop can add considerably more demand. Charging several phones at once may be modest in comparison, but the charging losses still count. If the laptop will mostly run from its internal battery, the backup system may only need to recharge it periodically; if it must operate continuously, plan for its sustained charger demand.
It can be more efficient to keep a phone topped up from a power bank and reserve the mains-output UPS for the router and laptop. That is a planning choice, not a fixed rule: check the capacity and output limits of the power bank, and remember that its stated capacity may not equal the energy available at the USB socket.
What about using a petrol or diesel generator?
A generator can supply a router’s electrical load, but its low-wattage demand does not automatically make it a sensible solution. A combustion generator must be operated outdoors, well away from buildings, doors, windows and ventilation openings. Never run one indoors, in a garage, basement, shed or another enclosed or partially enclosed space because of the risk of carbon monoxide. The Health and Safety Executive’s carbon monoxide guidance explains the danger.
Generators may also have minimum operating loads, fuel and maintenance requirements, noise concerns and output characteristics that need checking. Do not connect a generator to household wiring or a consumer unit by improvising a cable or backfeeding arrangement. A fixed connection, changeover system or other permanent installation should be designed and installed by an appropriately qualified professional.
For just a router and a few personal devices, a suitable battery system is often simpler to position and operate. The right choice depends on the required outage duration, the full load, budget and whether you need ordinary mains sockets or compatible low-voltage outputs.
Common sizing mistakes
Buying by watts alone. A high watt rating does not tell you how long a battery will last. Check Wh, usable energy and stated runtime as well.
Counting only the router. A separate ONT, modem or mesh node can be essential to the connection. Include each one.
Using the adapter rating as exact consumption. The label helps establish electrical compatibility, but it may not represent typical demand. Manufacturer data or a careful measurement is better.
Ignoring conversion losses. A battery feeding an inverter, then a router’s AC adapter, loses energy at each conversion stage.
Assuming internet access is guaranteed. Keeping your home equipment powered does not ensure that the wider broadband or mobile network is operating during an outage. This article focuses on your premises’ backup power, not the service provider’s network availability.
Frequently Asked Questions
How many watts does a broadband router use?
There is no single figure for every router. Check the power adapter and the manufacturer’s specification, and include any separate ONT, modem or mesh equipment. A measurement of the complete setup can give a more useful planning figure than a generic estimate.
How long will a UPS run a Wi-Fi router?
It depends on the UPS’s usable watt-hours, the combined load and conversion efficiency. Divide usable energy by the total load for a first estimate, then compare that result with the manufacturer’s runtime information at a similar load. Actual runtime can vary with battery condition, temperature and operating limits.
Can I run my router from a power bank?
Only if the power bank provides a suitable voltage, connector and polarity, or if a compatible manufacturer-approved converter is used. USB output alone does not make it suitable for a router. Check the router’s input requirements and the power bank’s continuous output rating before connecting anything.
As a practical next step, write down the input details for your router, ONT and the devices you want to charge, then enter their combined requirements into the UK Backup Power Calculator. That will give you a more defensible starting point for choosing capacity and runtime.