Router UPS Runtime Calculator

Why your internet dies when the power goes

The fiber line into your home needs no power at your end — but the two little boxes inside your house do. The ONT or modem that talks to your provider and the router that spreads Wi-Fi both go dark within seconds of a power cut. No Wi-Fi, no work calls, no streaming, no smart-home anything. A small UPS — or even a power bank with the right output — keeps both boxes alive through the outage. The question everyone asks is always the same: for how long? That is exactly what this calculator answers, from three numbers you can read off a label.

How the runtime math works

A battery's capacity is measured in watt-hours (Wh): one watt of load for one hour. Divide usable capacity by your total load in watts and you get hours of runtime. Real UPS units lose about 15% to conversion heat, so the calculator first multiplies capacity by 0.85 — the number you get matches real life, not a lab bench. Formula: runtime = (capacity in Wh × 0.85) ÷ (router watts + modem watts). It also works backwards: the capacity you need = total load × outage hours ÷ 0.85, so you can size a UPS before you buy one.

Worked example

Take a typical home setup: a router drawing 12 W and a fiber ONT drawing 6 W — 18 W total — on an 88 Wh mini-UPS. Usable energy is 88 × 0.85 = 74.8 Wh, so runtime is 74.8 ÷ 18 = 4.16 hours, about 4 h 9 min. A 2-hour load-shedding slot is covered with roughly 2.2 hours to spare. Now try a hungrier setup: a 20 W gaming router plus a 10 W modem (30 W) on a small 36 Wh unit — usable 30.6 Wh gives barely 1.02 h, about 1 h 1 min. For a 3-hour outage that falls about 2 hours short; you'd need roughly a 106 Wh UPS to ride the whole thing out.

Practical tips

Read watts straight off the power adapter label — watts = volts × amps, so a 12 V / 1 A brick means 12 W. Most home routers draw 5–20 W and fiber ONTs around 6 W. A 20,000 mAh power bank holds roughly 74 Wh (20 Ah × 3.7 V), but a plain 5 V USB bank cannot power a 12 V router — you need a bank with a 12 V DC output or a USB-C PD trigger cable. Keep the PC and monitor off the little UPS, or runtime collapses. And once, just test it: unplug the mains and time how long the Wi-Fi really stays up.