What Size UPS Do I Need for a Router and Modem?

Size a UPS by checking both its output limit and its published runtime at your equipment's measured watt draw. A 600VA or 1500VA label alone cannot tell you how many hours your internet will stay on.

Power station capacity for 4, 8 or 12 hours

These are planning estimates using the same preset wattages and calculation as our calculator. They apply to rated-capacity power stations, not traditional UPS batteries. Presets are starting assumptions: measure your own equipment and adjust the calculator.

The calculator allows for usable capacity, conversion losses and a safety margin. It does not model each power station's idle draw or low-load shutoff, which can matter greatly for a router. These estimates are not measured runtimes. See the calculation and its assumptions.

A published UPS example

These manufacturer-published points for the APC BE600M1 Back-UPS 600VA were retrieved on Sep 11, 2026. They show why load matters. They are not a measured result for your router. Manufacturers revise these curves, so check the current source before buying.

Published load	Published runtime, Sep 11, 2026
10W	258 minutes
40W	67 minutes
80W	30 minutes

Check the manufacturer's current runtime curve before buying. We do not calculate a precise runtime between these points or extend the curve below its published range. Compare other UPS models and their sources.

Choose the backup for the job

Start with two requirements: how long the equipment must run, and whether it must stay connected without rebooting. Runtime and transfer time are separate checks.

Requirement	What to check
Bridge a brief outage without a reboot	A UPS with sufficient output and a transfer time your equipment tolerates
Keep internet gear powered for several hours	Compare published UPS runtime at your load with a power station’s capacity and low-load performance
Run for 12 hours or longer	Check a larger power station or a UPS that explicitly supports extended runtime and compatible external batteries
Power a router directly from DC	Match voltage, polarity, connector and output current; capacity alone does not establish compatibility

There is no universal four-hour limit for a UPS. Battery configuration and load matter, and some models support external battery packs. Follow the manufacturer’s supported configuration. Do not chain UPS units together to extend runtime.

For the practical differences, read UPS vs Portable Power Station. For a mini DC UPS rated in mAh, see how long a mini UPS will run a router.

Measure everything that needs power

For cable internet, include the modem and router. For fibre, include the ONT and router. Add any network switches and mesh nodes that need to keep working. A combined modem/router is one device, so avoid counting it twice.

A plug-in power meter can measure the combined AC load. The number printed on a power adapter is its output rating, not necessarily what the connected device continuously consumes. Use your measured load to adjust the calculator’s presets.

Check that the UPS meets both the equipment’s watt and VA requirements, with headroom. Then check its runtime curve at that load. Watts vs Volt-Amps explains the two output ratings; neither measures stored energy.

Why battery watt-hours alone do not size a traditional UPS

Multiplying battery volts by amp-hours gives nominal stored energy. It does not establish how much a UPS delivers to the sockets at a particular load. Battery discharge behaviour, inverter overhead, cutoff settings and battery condition affect the result.

Do not apply a fixed percentage of battery capacity to every UPS. Use the exact model’s published curve, including its battery configuration. If the manufacturer only publishes half-load and full-load runtimes, those figures cannot establish runtime for a small router load. Read How Long Will a UPS Run? for the method.

What can interrupt an eight-hour setup?

Transfer time can cause a reboot. Standby and line-interactive UPS models have a transfer interval, as do many power stations. Check the exact specification and test whether your equipment remains connected when mains power is removed.

Low-load shutoff can stop a power station early. Some units turn off an AC output when the draw stays below a threshold. Check the manual for an appropriate always-on setting, and allow for inverter idle consumption.

Every remote box needs power. An ONT in a utility cupboard or a mesh node in another room needs its own power connection. Keeping only the main router on does not preserve every part of the network.

The ISP may lose power upstream. Local battery backup can preserve your home network even when the outside internet connection fails. It cannot guarantee that the provider’s equipment remains online.

Battery condition changes runtime. Check an older UPS battery’s condition and test the setup before relying on it. Leave margin for ageing and changes in equipment load.

Make the final comparison

Use the capacity links above for a power station estimate, then check the suggested model’s manual for low-load behaviour and transfer time. For a traditional UPS, use the published runtime comparison.

If you already own a battery and want to estimate duration, read How Long Can a UPS Run a Router and Modem?. For a laptop and monitor as well, see backup power for working from home.