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Keep the internet up through a power cut: sizing a mini UPS for your router and ONT

A mini UPS for networking gear is one of the few genuinely good cheap purchases in this whole area — provided you buy the right one, and provided you know what it can and cannot do. The "up to 8 hours" printed on the box is not a lie exactly; it is an answer to a question nobody asked. This guide gets you to the real number for your hardware, in about twenty minutes with a calculator.

What actually goes dark, in order

When the power goes, four things stop: the fibre ONT, the router, the TV box, and the TV. The ONT is the one people forget. Back up the router alone and it stays online cheerfully with nothing behind it — from the phone in your hand, that looks exactly like a dead connection. The ONT is where your internet comes from; it must be on the backup list.

The TV needs mains power no matter what you do to the network. So decide early which goal you are buying for:

• "The house keeps internet on phones and tablets." Router plus ONT. A mini UPS does this for a few thousand rupees.
• "The TV keeps playing." A different, much larger purchase. Be honest about which one you want before you shop.

Measure your load in two minutes

Every power adapter has its output printed on the label: a voltage and a current. Multiply them for watts.

Two important caveats. The nameplate is the adapter's maximum, not what the device draws in normal use — real consumption is usually well below it, so treating the total as a worst case gives you a safety margin rather than a disappointment. And add up only the devices you actually intend to back up. If the TV box is not on the list, its watts are not on the list either.

How long a mini UPS really lasts

Here is the mechanism behind the misleading number. Capacity on these units is quoted in mAh, and it is quoted at the cell's own voltage of about 3.7 V, not at the 12 V your router wants. To get to a number that means anything, convert to watt-hours:

Wh = mAh × 3.7 ÷ 1000

An 8800 mAh unit is therefore about 32 Wh of stored energy. Converting that stored energy up to 9 V or 12 V loses some of it. How much is not something this guide has measured, so it uses one rough allowance throughout — multiply the stored figure by 0.8 — which puts an 8800 mAh unit at about 26 Wh usable. Then:

Runtime in hours = usable Wh ÷ total watts

• At a combined 18 W: about 1.4 hours.
• At 10 W: about 2.6 hours.
• At 30 W: under an hour.

The "up to 8 hours" figure works out to roughly a 3 W load, which is less than almost any real router. Run the arithmetic with your own number from the section above, and buy for the length of a typical cut in your area plus a margin — not for the headline.

Buy the right output, not just the right capacity

Capacity is useless if the unit cannot physically power your gear. Check all of these before ordering:

• Voltages. Networking kit is commonly 9 V or 12 V DC, sometimes with a 5 V USB output as well. The unit must offer the voltage each device needs.
• Current per output. Each output's rating must meet or exceed the rating on that device's own adapter.
• Barrel connector size. 5.5 × 2.1 mm is the most common but it is not universal; measure or check before assuming.
• Polarity. Centre-positive on most networking gear. Getting this wrong destroys equipment.
• Never feed a device a higher voltage than its label states. A 9 V router on a 12 V output is not a shortcut.

If your equipment is powered over Ethernet rather than a barrel jack, a mini UPS of this type is the wrong product — you need to back up whatever is injecting the power instead.

The inverter you may already own

If the house has an inverter, the cheapest fix is often not a purchase at all: an electrician moves the router and ONT socket onto an inverter-backed circuit. Changeover is usually fast enough that a router rides straight through it. If yours reboots on every changeover, that is a real test failing, and a small UPS in front of the router solves that specific problem.

Compare the two honestly. The wiring change usually wins on cost and runtime where the inverter already exists and the socket is reachable. The standalone mini UPS wins in a rented flat, where you want no electrician and nothing modified, and where you may move house next year.

Watching something during the cut

The TV is by far the biggest load in the room and a mini UPS will not come close to running it. The practical answer during a cut is to continue watching on a phone or tablet — which is exactly why keeping the router and ONT alive matters more than keeping the TV box alive. Casting is pointless when the TV is off.

Note that if the cut also takes out the tower or your line, the phone will fall back to mobile data, and then the arithmetic in the hotspot data guide becomes the relevant constraint — two hours of a cut at 3 Mbps is 2.7 GB.

If you genuinely want the TV running through cuts, that is a full inverter sized to the TV's own wattage plus the network gear, which is a different order of purchase. Read the TV's rating label and be realistic: a set drawing 100 W needs more stored energy in one hour than an 8800 mAh mini UPS holds in total, three times over.

Protecting the gear when power returns

Surges on restoration are a real risk to the adapters, and a spike guard on the mains side of them is cheap insurance. One genuine advantage of fibre: the fibre strand carries light, not current, so it cannot conduct a surge into your ONT the way copper telephone or cable runs can. Your exposure is the mains side only.

A worked example to copy

• Router 10 W + ONT 6 W = 16 W total load.
• 8800 mAh × 3.7 ÷ 1000 = 32.6 Wh stored; × 0.8 = roughly 26 Wh usable.
• 26 ÷ 16 = about 1.6 hours.

Now the blank version. Fill it in from your own adapter labels before you buy anything:

1. Router ___ W + ONT ___ W (+ box ___ W, if you are backing it up) = ___ W total.
2. ___ mAh × 3.7 ÷ 1000 = ___ Wh stored; × 0.8 = ___ Wh usable.
3. Usable Wh ÷ total W = ___ hours.

If line three is shorter than a typical cut where you live, you need a larger unit — not a different brand of the same size.