Size your smart home backup battery by runtime, not wattage

If you are buying smart-home backup battery protection for a power outage for a modem, router, hub, and a few cameras, the first sizing question is not “How many watts can this box handle?” It is “How long can this battery feed my small network load before everything starts falling offline?”

That distinction matters because a smart-home network core is usually a small, continuous load. The scary-looking wattage limit on a UPS is rarely the part that decides whether your thermostat, Zigbee lights, cameras, and phone alerts survive a neighborhood outage. Battery capacity at 25–40W is the part that decides whether you get minutes or hours.

Add the devices first, then choose the battery

Start with the equipment that must stay online for the smart home to keep behaving like a system: the modem or ONT, the router, the main access point if it is separate, and the hub or radio coordinator. If those die, the outage is not just an internet problem. It can become a recovery problem, with cameras offline, automations missed, and devices that have to be coaxed back in the right order later.

The draw numbers below are useful planning estimates from HomeTechHacker’s smart-home UPS guide, not lab measurements of your exact hardware. If your adapter label, power meter, or UPS software gives you a measured number, use that instead.[1]

A bare but useful stack — modem, router, and hub — lands around 21–33W using those estimates. Add a separate access point and you are roughly in the 29–45W range. That is why the practical target for many homes is not a giant wattage number; it is enough battery to carry a roughly 25–40W network core for the outage window you care about.[1]

The load can climb if your smart home depends on a PoE switch, an NVR, or a NAS. But those are not the same kind of load as the modem and router. A NAS may be important for file integrity or local recording, but it is not automatically more important than the hub that keeps your sensors and automations from dropping into a messy restart.

Runtime data punctures the VA-number anxiety

UPS boxes are sold with VA ratings because the unit has to be able to supply a certain amount of power. But for a modem-and-router load, the UPS is not usually being pushed anywhere near its output ceiling. The useful comparison is runtime at a small load.

Those numbers should not be read as a promise that your closet will get the same runtime. Your devices may draw more than 20W, your UPS may be older, and batteries age. But the shape of the decision is clear: a 20W network load is a completely different sizing problem from a 300W desktop-PC load.[2]

This is also why converting VA into a false runtime answer is such a bad shortcut. VA tells you something about the UPS’s output capability. It does not tell you, by itself, how many minutes your modem, router, coordinator, switch, and cameras will stay alive.

A runtime-first sizing workflow

Do the sizing in this order, and the shopping list gets smaller fast.

1. Write down the devices that must stay online: modem or ONT, router, access point, hub or coordinator, switch, cameras, NVR, and NAS.
2. Add their watts. Use measured draw if you have it; otherwise use the planning ranges above and round up.
3. Choose the runtime target before you shop. If you do not know whether you are planning for short blinks or regional storm outages, use the site’s outage-duration planning guide first.
4. Compare UPS models by runtime at a load close to your total, not by VA alone.
5. Add optional devices only if the runtime still meets the target.

For a typical 25–40W core, the 850–1000VA class is the first shelf I would compare for most homes. It is large enough to be meaningfully different from the smallest UPS units, small enough to live near a network closet, and usually priced below the point where you should be asking whether you actually meant a portable power station or a whole-home backup system.

If your total is closer to 60W because you added a switch and a camera bridge, the same UPS will run for less time. If you add a NAS, do the math again from scratch. A NAS can be a perfectly valid thing to protect, but it should not silently steal half the battery from the devices that keep the rest of the house reachable.

If what you really want is refrigerator coverage, lights, medical gear, garage-door operation, or multi-hour appliance backup, that is a different tier. Use the broader smart-home backup power sizing guide or the whole-home and portable-station guide. A UPS is the right tool for a low-power network core, not a miniature house battery.

Plug-in priority: protect the failure cascade, not the fanciest device

Most UPS units have a limited number of battery-backed outlets. Some outlets may be surge-only. The battery-backed side should go to the devices that prevent the most annoying recovery work.

1. Modem or ONT. Without this, the router may still make Wi-Fi, but the home is cut off from the internet.
2. Router or main gateway. This keeps local routing, Wi-Fi control, DHCP, and many local integrations alive.
3. Smart-home hub, Zigbee coordinator, Z-Wave controller, or Thread border router. This is often a tiny load with an outsized recovery benefit.
4. Access point or switch, if your Wi-Fi or hub depends on it.
5. Essential cameras, if the added draw still leaves enough runtime.
6. NVR or NAS, only after you have checked the new total load against the runtime target.

That third item is easy to undervalue because it draws so little power. It is also the one that can save you from the most tedious after-outage cleanup. If the coordinator never drops, you are less likely to spend the next hour wondering why half the room came back before the radio mesh was ready. For the deeper failure-cascade explanation, use the smart devices offline after power outage troubleshooting guide; for this purchase decision, the rule is simple: keep the network brain powered before you spend battery on recording or storage.

Cameras deserve a stricter test. A camera that stays powered while the router dies may record locally, depending on the system, but it will not necessarily give you the alert you wanted. A camera that cuts your network runtime too far may also be the reason the hub and router die before the outage is over. Plug in one or two critical cameras only after the modem, router, and hub are covered.

Do not treat a UPS like a tiny wall circuit

A UPS belongs directly in a wall outlet. Wirecutter’s UPS safety guidance says not to plug a UPS into an extension cord or a surge protector, and not to run space heaters, vacuums, or sump pumps from battery-backed outlets.[2]

The same common-sense boundary applies to appliance loads. If it heats, pumps, spins a motor, or can suddenly demand a lot of current, it is not part of this smart-home network-core UPS plan. Put the modem, router, hub, and related low-power network gear on the UPS. Leave the rest of the house to the right backup tier.

Features worth filtering for after you know the runtime target

Once your load and runtime target are clear, the remaining shopping filters are practical. They should remove bad fits; they should not tempt you into buying wattage you will never use.

Lead-acid versus lithium should come after those basics, not before them. A lithium UPS or battery product may be appealing, but chemistry does not rescue a poor outlet layout, a missing monitoring port, or a runtime curve that does not fit your load. For this layer, a boring UPS with enough battery-backed outlets, replaceable battery support, and runtime close to your target can be the better buy.

If you want model-by-model recommendations rather than sizing logic, use the site’s smart-home power-outage gear guide after you have your load number. Runtime expectations make product roundups much easier to read.

Monitoring comes after the purchase

A UPS can also become an outage sensor. Home Assistant’s NUT integration connects Home Assistant to Network UPS Tools so UPS status can be exposed inside the smart home.[3]

That is useful after the hardware decision is made. You can build alerts, log outages, or trigger graceful automations with the UPS power-outage sensor recipe and the broader power-outage automation guide. Just do not let monitoring distract from the battery math. An automation can tell you the UPS is draining; it cannot make an undersized battery run longer.

The purchase rule

For most smart-home network cores, buy enough battery for the runtime you want, not excess wattage you will never use. Start with the modem, router, and hub or coordinator. Add the access point or switch if the network depends on it. Add cameras, NVR, or NAS only if the runtime math still works.

If your load is roughly 25–40W, compare 850–1000VA UPS models first and check measured or manufacturer-provided runtime at a similar load. If your list has grown into appliances, long-duration lighting, or whole-home coverage, stop shopping UPS towers and move to the larger backup-power tier.

References

1. How to Choose the Right UPS for a Smart Home — HomeTechHacker
2. The 3 Best Uninterruptible Power Supplies (UPS) of 2026 — Wirecutter
3. Network UPS Tools (NUT) — Home Assistant