--- title: "UPS Runtime Calculator for a Router and Modem | Watt Cottage" description: "UPS runtime calculator: minutes of backup for your router and modem from the battery's watt-hours or the maker's runtime chart, with VA vs watts explained." url: https://wattcottage.me/ups-runtime-calculator/ --- UPS runtime calculator # How long will a UPS keep my router and modem on? Multiply the UPS battery’s watt-hours by the share the UPS can use and by its inverter efficiency, divide by the load in watts, and multiply by 60 for minutes. A router and a modem drawing 40 W between them, on a UPS with a 12 V 9 Ah battery (108 Wh), get **about 2 hours 18 minutes** at 85% efficiency. If the maker publishes a runtime chart for your model, enter two points from it: it was measured on that UPS, so trust it over the sum. ## How the estimate works A UPS (uninterruptible power supply) is a battery with a charger and an inverter in one box. When the mains fails, the inverter turns the battery’s DC into AC for whatever is plugged into the battery-backed outlets. The calculator works the runtime out two ways. The battery estimate is energy ÷ load: watt-hours (volts × amp-hours, so 12 V × 9 Ah = 108 Wh) × the share the UPS can use × the inverter’s efficiency, divided by the watts plugged in. The chart estimate uses two points you copy from the maker’s runtime chart and draws the curve through them: runtime = a × loadb, a straight line on log-log paper, which follows a runtime chart’s shape far more closely than a straight line between the two points does. When both are there, the chart wins. It was measured on that model, so it already includes whatever the UPS itself uses and how its battery behaves at that load, which the simple sum can’t know. Use points either side of your load; outside them the curve is a rough guide only. ## VA versus watts A UPS label gives two ratings, such as 600 VA and 360 W. Volt-amperes (VA) are volts × amps, the apparent power the electronics have to handle. Watts are the real power your devices turn into light, heat and computing. For many electronic loads the watts are lower than the volt-amperes, so the watt rating is the smaller number and the one to check your load against: keep the total watts plugged in below it. Victron’s inverter datasheet shows the same pattern: its Phoenix 12/375 is rated 375 VA but 300 W continuous at 25 °C. No wattage on the device? The OUTPUT line on its adapter gives the most it can supply: volts × amps. Using it errs on the safe side. ## Keeping the internet up The *Keep the internet up* button loads the usual pair: the Wi-Fi router and the modem or fibre box (ONT) that feeds it, at 20 W each. That is the top of Champion’s 10 to 20 W range for a router or modem; Electrical Safety First lists a Wi-Fi router at 15 W. If your router and modem are one box, delete the second row. Anything else in the chain needs power too: a mesh node, a network switch, or a cordless phone base (Electrical Safety First: 10 W). Ready.gov suggests finding out whether your home phone will work in an outage and how long its battery backup lasts, and FEMA reminds us that landlines won’t work if the lines are damaged. A UPS keeps your own equipment on; whether the connection works also depends on your provider’s network. ## A worked example The example UPS is rated 600 VA and 360 W, with a 12 V 9 Ah battery. 12 × 9 = 108 Wh; at 100% usable and 85% efficiency that is 91.8 Wh delivered. The router and modem draw 40 W, 11% of the watt rating, so the battery estimate is 91.8 ÷ 40 = 2.3 hours, 137.7 minutes: about 2 hours 18 minutes. Add two points from the maker’s chart and the result switches to the chart’s answer. If 40 W lies below the lowest load you copied, the meter says so: the curve is then a rough guide, so pick the chart’s lightest load as one of your two points. ## Common mistakes - **Sizing by VA.** Compare your watts with the UPS’s watt rating, the smaller of its two numbers. - **Using the wrong outlets.** If your UPS has outlets marked as surge protection only, they get no battery power: plug the router into a battery-backed one. - **Trusting an old battery.** Like any battery, a UPS battery holds less as it ages; lower the usable share for an old one. - **Adding a heater or a printer.** Big loads cut the minutes to a fraction. Keep the battery outlets for what must stay on. ### Runtime chart behind an email form? Some makers and shops ask for an email address before you can download a brochure or runtime chart, or to send a one-off discount code. A temporary inbox from [CleanTempMail](https://cleantempmail.com/) receives the download or code with no sign-up and empties itself after three days, so the newsletters that follow go nowhere. Register the UPS for its warranty with your real address, because that is where recall notices and warranty replies go. A UPS is for minutes or a few hours. For a night without power, a portable power station holds far more; compare with the [power station runtime calculator](https://wattcottage.me/power-station-runtime-calculator/). If someone relies on powered medical equipment, follow the device maker’s guidance rather than an estimate like this one. For the UPS’s own running cost, Lightnest Market’s [standby power cost calculator](https://lightnestmarket.shop/standby-power-cost-calculator/) works out what an always-on box costs a year. ## Sources All checked on 5 November 2026. - Victron Energy: [Phoenix Inverter VE.Direct datasheet, 250 to 1,600 VA](https://www.victronenergy.com/upload/documents/Datasheet-Phoenix-Inverter-VE.Direct-250VA-800VA-EN.pdf) (VA and watt ratings, maximum efficiency, zero-load power). - Champion Power Equipment: [Generator wattage chart](https://www.championpowerequipment.com/generator-wattage-chart/) (internet router/modem, 10 to 20 W). - Electrical Safety First: [Ratings of electrical appliances](https://www.electricalsafetyfirst.org.uk/safety-advice/home-and-people/house-maintenance/ratings-of-electrical-appliances/) (Wi-Fi router, cordless phone charger, computers). - Ready.gov: [Power outages](https://www.ready.gov/power-outages) (last updated 4 June 2026) and FEMA: [Be prepared for a power outage](https://www.ready.gov/sites/default/files/2024-08/ready.gov_power-outage_info-sheet.pdf) (FEMA V-1008, December 2023). ## Questions **How long will a UPS run a router and modem?** Battery watt-hours × usable share × efficiency ÷ load. With a 12 V 9 Ah battery (108 Wh), 85% efficiency and 40 W of router and modem, about 2 hours 18 minutes. A 7 Ah battery of the same voltage gives about 1 hour 47 minutes. The maker’s runtime chart is better than either estimate. **What is the difference between VA and watts on a UPS?** VA is volts × amps, the apparent power the UPS handles; watts are the real power your devices use. The watt rating is usually the smaller figure and the one to compare your load with. A 600 VA, 360 W UPS can carry up to 360 W. **How do I find a UPS battery’s watt-hours?** Multiply the battery’s volts by its amp-hours, both printed on it: 12 V × 9 Ah = 108 Wh. If the UPS has two 12 V batteries in series, that is 24 V at the same amp-hours. The spec sheet may list the battery’s watt-hours directly. **What size UPS do I need for a router?** Almost any UPS can carry a router’s watts; what differs is the runtime. Decide how long you want it to last, then work back: minutes ÷ 60 × load ÷ efficiency gives the watt-hours you need. Two hours for 40 W at 85% needs about 94 Wh. **Will a UPS keep my Wi-Fi working in a power cut?** It keeps your router and modem powered for as long as the battery lasts. Whether you can still reach the internet also depends on your provider’s network having power, which a home UPS can’t control.