Quick answer
With the default example, a 100 Wh backup battery powering a 22 W router plus modem load gives about 3 hr 29 min of estimated runtime after efficiency and a small reserve.
How to use this router backup power runtime calculator
Start with the battery capacity. If the label gives watt-hours (Wh), use that directly. If it gives milliamp-hours (mAh), enter the battery voltage shown on the battery label, not the router output voltage. Many USB power banks list mAh at their internal cell voltage.
Then add the load. A home internet backup often needs more than the router: modem, fiber ONT, gateway, Ethernet switch, or mesh nodes may also need power. Leave TVs, monitors, laptops, and chargers off this backup unless you intentionally want a much shorter runtime.
If your problem is coverage rather than outage runtime, use the Mesh Router Coverage Calculator or the Wi-Fi Extender Placement Calculator instead.
Router backup runtime formula
battery Wh = mAh x battery voltage / 1000total load watts = router W + modem/ONT W + mesh node W + extra Wusable Wh = battery Wh x efficiency x battery condition x (1 - reserve %)runtime hours = usable Wh / total load wattsThe formula uses watt-hours because runtime is an energy question. UPS VA rating, adapter current rating, and marketing battery size can be useful labels, but they do not directly answer runtime unless you convert them into usable Wh and actual load W.
Assumptions and methodology
This calculator is built for practical home backup planning. It assumes the battery can deliver the required output and that the router, modem, ONT, or mesh nodes stay within the battery device's output rating. It does not replace a manufacturer UPS runtime chart for a specific UPS model.
- AC UPS and power-station outputs include conversion losses, so usable energy is lower than nominal battery Wh.
- DC mini UPS devices can be more efficient, but voltage, polarity, and connector compatibility matter.
- Older lead-acid UPS batteries may deliver much less than their original capacity, especially after heat exposure or years of standby use.
- Internet may still fail during an outage if upstream ISP equipment or local fiber/cable infrastructure loses power.
Example calculations
Router and modem backup runtime example
Suppose your router uses 12 Wand your modem or ONT uses 10 W. The total network load is 22 W.
With a 100 Wh backup battery, the calculator estimates 76.5 Wh usable energy after efficiency and reserve. 76.5 Wh usable battery divided by 22 W load gives about 3 hr 29 min of backup runtime.
If you add mesh satellites, an Ethernet switch, or a small NAS to the same backup battery, total watts increase and runtime drops. For outage planning, keep only the network devices required to keep internet service online.
Router backup runtime chart
These examples show why the same battery size can produce very different runtime depending on whether it powers only a router or a full modem, ONT, and mesh setup.
| Setup | Battery | Load | Usable | Runtime | Guidance |
|---|---|---|---|---|---|
| Small DC mini UPS, router only | 36 Wh | 10 W | 29.8 Wh | 2 hr 59 min | Short outage coverage |
| Small UPS, router + modem | 100 Wh | 22 W | 76.5 Wh | 3 hr 29 min | Short outage coverage |
| 150 Wh backup, router + ONT | 150 Wh | 22 W | 124 Wh | 5 hr 39 min | Useful outage coverage |
| 256 Wh power station, mesh home | 256 Wh | 40 W | 203 Wh | 5 hr 4 min | Useful outage coverage |
| 512 Wh power station, longer outage | 512 Wh | 40 W | 406 Wh | 10 hr 8 min | Long outage coverage |
What should be on router backup power?
Put only the essential network chain on backup power: router, modem, ONT, gateway, and possibly one mesh node if the router location does not cover your work area. Avoid plugging monitors, TVs, desktop PCs, game consoles, chargers, or speakers into the same battery unless the goal is a much shorter runtime.
For a DC mini UPS, verify output voltage, maximum current, connector size, and polarity before connecting equipment. For an AC UPS, avoid overloading the battery with non-network devices and replace aging batteries before relying on them for emergency internet.
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FAQ
How long will a UPS run a router?
It depends on the UPS battery watt-hours and the total load in watts. A small 100 Wh UPS with about 76 Wh usable after efficiency and reserve can run a 22 W router plus modem load for roughly three and a half hours.
Can I run a modem and router from the same UPS?
Yes, if the UPS output wattage is enough and the battery capacity is large enough. Add the router watts, modem or ONT watts, and any mesh node watts to estimate total load before calculating runtime.
Does UPS VA rating tell me router runtime?
No. VA rating tells you power capacity, not stored energy. Runtime depends on the battery capacity in Wh, device load in W, efficiency, battery age, and reserve. Manufacturer runtime charts are still useful when available.
Is a DC mini UPS better than an AC UPS for a router?
A DC mini UPS can be more efficient for routers because it may avoid converting battery DC to AC and then back to DC through the router adapter. It must still match the router or ONT voltage, current, and connector.
How many watts does a router use?
Many home routers use roughly 8-20 W, but mesh routers, Wi-Fi 6E or Wi-Fi 7 systems, and gateway devices can use more. The best value is the wattage listed on the power adapter or measured with a plug-in power meter.
Will my internet work during a power outage?
Your router and modem may stay on if they have backup power, but your internet provider's outside equipment also needs power. Fiber ONT, cable node, ISP cabinet, and local network conditions can affect whether service remains online.
Can I use a USB power bank for a router?
Sometimes. The power bank must provide the correct output voltage and enough current, often through USB-C PD or a suitable DC converter. Do not connect a router to a random cable unless the voltage and polarity match.
Is this router backup runtime calculator exact?
No. UPS batteries, inverter losses, battery aging, low-load behavior, temperature, and device power draw all vary. Use the calculator as a planning estimate, then test the backup setup before relying on it during an outage.