UPS runtime calculator: how long will it run your router?

How the calculation works

Runtime comes from three steps. First, add up the load in watts. Second, work out how much of the battery your equipment can actually use. Third, divide.

• Load (W) = modem or ONT + router + hub + anything else you plug into the backup.
• Delivered energy (Wh) = battery capacity (Wh) × usable share ÷ 100 × efficiency ÷ 100.
• Runtime (hours) = delivered energy ÷ load. The same watts-and-hours relationship the Department of Energy uses for appliances: watts × hours ÷ 1,000 gives kilowatt-hours.

Example 1. Modem 8 W + router 10 W + hub 4 W = 22 W. A small desktop UPS with about 100 Wh, 80% usable and 85% efficiency delivers 100 × 0.80 × 0.85 = 68 Wh. 68 ÷ 22 = 3.09 hours, which the calculator shows as 3 h 5 min.

Example 2. The same 22 W on a 250 Wh portable power station: 250 × 0.80 × 0.85 = 170 Wh. 170 ÷ 22 = 7.73 hours, shown as 7 h 44 min.

Sizing in reverse. To keep 22 W running for 12 hours with the same assumptions, you need 22 × 12 ÷ (0.85 × 0.80) = 388 Wh of battery, so shop for roughly 400 Wh or more. The calculator’s “battery needed” row does this for any target time you enter.

Where the numbers come from

Wattage. Look at the label on each device’s power adapter. It lists volts and amps: multiply them for watts. The label shows the maximum, and real use is usually lower. A plug-in power meter or an energy-monitoring smart plug gives the real figure; our device energy cost calculator uses the same method.

Efficiency. A battery that powers a 120 V outlet has to convert DC to AC and the devices convert it back, so some energy is lost as heat. About 85% is a reasonable planning figure for an AC UPS or a power station, and it is an assumption, not a measured value for your model. A mini DC UPS that powers your router directly at 9 V or 12 V wastes less. Check that its output voltage and polarity match your equipment exactly.

Usable share. The calculator keeps a 20% reserve by default because batteries lose capacity as they age, and cold weather reduces it further. Lower the number for an old battery.

VA, watts and watt-hours are three different things

A UPS rated for a desktop computer is built to carry it for a few minutes, not to run a router for a day. Battery size varies a lot for a given VA rating, so always look for the battery’s Wh, or its voltage and Ah, and compute the rest.

Common mistakes

• Forgetting a device. The modem, the router, every mesh node that needs power, a switch and a hub all count. If a mesh node is on wall power but not on the backup, the network falls apart.
• Backing up a Wi-Fi router but not the modem. You will have Wi-Fi with no internet. Local control still works (see what your smart home does without internet).
• Assuming your provider stays up. A UPS keeps your equipment powered. The cable node, fiber cabinet or cell tower that serves you can lose power as well.
• Using the VA rating as a runtime guide. See the table above.
• Skipping battery safety. Use only the charger the maker supplies, charge in a cool dry place, never leave lithium-ion packs charging unattended and replace swollen or damaged batteries. Look for a recognized safety certification such as UL.
• Using the backup for medical equipment. A router UPS is not designed for devices people depend on for health. Talk to your doctor and the equipment supplier, and read our power outage backup kit for the planning questions to ask.