Last time we unplugged the internet and checked what survived. That test proves the architecture, but it skips the more common failure: the storm that takes out the line usually takes out the mains too, and a recorder with no battery behind it doesn’t get to demonstrate how local it is. It just stops.
This post is the sizing math for a small UPS, done with numbers you can check against your own gear. It is not complicated, but almost everyone who skips it ends up buying either a unit that lasts four minutes or a unit twice the size they needed.
First, decide what the UPS is for
A UPS in a camera closet does one of two jobs, and the sizing is completely different depending on which one you pick.
Job 1: ride through short blips. Most residential outages in a storm are seconds to a few minutes. If the battery covers that, the recorder never notices, and you avoid the thing that actually damages footage: a hard power cut mid-write, followed by a reboot, a disk check, and a camera reconnect dance. Ten to fifteen minutes of runtime covers the large majority of these.
Job 2: keep recording through a long outage. An hour or more. This is a different purchase, usually a larger unit or an external battery pack, and the honest question is whether the cameras will have anything to see. Your router and modem also need power, and the upstream box the fiber or cable connects to may be dark anyway.
Most homes want job 1 plus one extra feature: enough runtime for the recorder to shut itself down cleanly before the battery empties. That combination is cheap, and it’s what the rest of this post sizes for.
What actually needs battery
Not everything in the rack. Put the UPS behind:
- The recorder. The mini-PC or NVR running Frigate. This is the only device where an unclean power loss can cost you data.
- The PoE switch. No switch, no cameras. This is also, by far, the biggest load, which surprises people.
- The router, if you want local viewing to keep working during the outage. Tiny load, worth it.
- The modem or fiber terminal, only if you want push notifications and remote viewing to have a chance. Also a tiny load, but remember the upstream side may be down regardless.
Leave off the UPS: computer screens, desktop PCs, anything with a speaker, anything you plugged in “while you were there.” Every extra watt is runtime you paid for and gave away.
The watt math
Runtime is set by watts drawn, not by the VA number in the product name. Work in watts.
Step 1: the recorder. A low-power mini-PC running Frigate with a USB detection accelerator sits in the single digits to low teens of watts at idle and roughly doubles under sustained detection load. A repurposed desktop is a different animal: 40 to 100 watts is normal. If you don’t know, this is the one device worth measuring with a plug-in power meter for a day, because it’s the only figure you can’t get from a label.
Step 2: the cameras, via the switch. Look up each camera’s spec-sheet power figure. Fixed PoE cameras from the major brands typically list a maximum in the 6 to 12 watt range. Cameras with spotlights, sirens, heaters, or pan-tilt motors list more, sometimes a lot more, and the spotlight matters because it comes on at night, which is exactly when the outage happens.
Add those maxima up. That is your PoE draw ceiling. Real draw is lower most of the time, but you size for the ceiling because a cold night with every spotlight lit is a real condition, not a theoretical one.
Then add the switch’s own overhead. A 16-port PoE switch burns a handful of watts running itself before it powers a single camera, and PoE conversion isn’t free, so budget roughly 10 to 15 percent on top of the camera total for losses.
Step 3: the small stuff. A consumer router and a modem together are usually 10 to 25 watts. Add them if they’re on the UPS.
Step 4: add it up and add margin. Total the above and multiply by 1.25. Batteries age, and a unit sized to exactly today’s load will be undersized in three years.
A worked example, using the shape of our own install (ten fixed PoE cameras, an N100 mini-PC, one 16-port PoE+ switch, router and modem), with spec-sheet maxima rather than measured averages:
| Load | Watts (ceiling) |
|---|---|
| Mini-PC with detection accelerator, under load | ~15 |
| 10 cameras at ~10 W spec max each | ~100 |
| Switch overhead and PoE losses (~12%) | ~15 |
| Router + modem | ~15 |
| Subtotal | ~145 |
| With 25% margin | ~180 |
Real average draw on a setup like this is meaningfully lower than the ceiling, because cameras don’t sit at their spec max all night. But 180 watts is the number to shop with.
Reading the UPS box
Two numbers matter and neither is the one in the model name.
Watt rating. An “850 VA” consumer UPS is typically rated for around 500 watts of actual load. Our 180-watt design load fits comfortably. A 1500 VA unit is usually rated near 900 watts, which is far more capacity than this load needs unless you want long runtime.
Runtime at your load. Manufacturers publish runtime at half load and full load, and those figures are short: a common 850 VA unit lists something like nine minutes at half load and three minutes at full. That sounds bad until you notice that half load is 255 watts and you’re drawing 180 or less. Most vendors also publish a runtime chart or calculator per model; find it and read the row nearest your watt figure. For a sub-200-watt load on an 850 VA class unit, you’re generally in the 12 to 20 minute range. On a 1500 VA class unit, roughly double that.
That is enough for job 1 and enough for a clean shutdown. If you want an hour, you’re shopping for a unit with external battery packs, and the price roughly triples.
Waveform. Get a pure sine wave model if the recorder has an active PFC power supply, which most modern mini-PC bricks and desktop supplies do. Simulated sine wave units can make those supplies buzz or drop out when the UPS switches to battery, which defeats the purpose. The price difference is small.
Check the PoE budget while you’re here
This isn’t a UPS question, but it’s the same math and people get bitten by it in the same closet.
Every PoE switch has a total PoE budget, separate from its port count. A 16-port PoE+ switch might advertise 30 watts per port but carry a total budget in the neighborhood of 190 to 250 watts depending on the model. Sixteen ports times 30 watts is 480. The switch cannot deliver that, and it doesn’t pretend to; it’s in the datasheet.
Add up your camera maxima the same way you did above. If the total is within about 80 percent of the switch’s budget, you’re fine. If it’s over, the switch will start dropping power to the lowest-priority ports under load, and the symptom is a camera that mysteriously reboots on cold nights when the spotlights and heaters kick in. That gets diagnosed as a bad camera or a bad cable far more often than it gets diagnosed as a PoE budget problem.
Ten fixed cameras at a 10-watt ceiling is 100 watts, which sits comfortably inside any 16-port PoE+ switch’s budget. Ten cameras where four of them are PTZ units with heaters is a different sum. Do it once.
Make the recorder shut down cleanly
This is the part most people skip, and it’s the part that turns a UPS from “buys a few minutes” into “actually protects the data.”
Nearly every consumer UPS has a USB port that reports battery state to whatever it’s plugged into. Plug it into the recorder, and run a small daemon that watches it:
- NUT (Network UPS Tools) is the standard on Linux and supports nearly every brand. It’s in every distro’s package repo.
- apcupsd is the older, APC-focused option and still fine if that’s your brand.
The configuration you want is simple: when the UPS reports it’s on battery and the remaining charge drops below a threshold (say 30 percent, or a fixed runtime like five minutes), the daemon issues a normal system shutdown. Frigate, running in Docker, gets a stop signal, finishes the segment it’s writing, and exits. The disk unmounts. The machine powers off with battery to spare.
Two details that matter:
Order the stop. Docker sends a stop signal and waits a default ten seconds before killing the container. Frigate can need longer than that to close out recordings cleanly. Set a longer stop grace period on the Frigate container (30 seconds is a reasonable value) so the daemon’s shutdown doesn’t turn into the very hard kill you bought the UPS to prevent.
Test it on purpose. Once it’s configured, pull the UPS’s wall plug on a quiet afternoon and watch. Confirm the daemon notices, confirm the shutdown fires at the threshold you set, and confirm the recorder comes back on its own when power returns (the BIOS “restore on AC power loss” setting, which usually defaults to off). Then check the timeline for a clean gap with no corruption warnings on either side. If you don’t run this test, you don’t have a shutdown setup; you have a config file.
What you don’t get
A UPS behind the recorder does not:
- Keep the internet up if the outage extends past your house. Local recording and local viewing survive; anything remote depends on gear you don’t own.
- Guarantee notifications. Same point we make every time: push is best-effort on any camera system, and a battery on the recorder doesn’t change how the message gets to your phone.
- Replace surge protection. Most consumer UPS units include some, but a lightning strike on an outdoor camera cable comes in through the ethernet, not the mains. Inline ethernet surge protectors on outdoor runs are a separate, cheap purchase and the one people most regret skipping.
- Last forever. The battery in a consumer unit is a consumable with a life of roughly three to five years. Put a reminder somewhere.
The short version
Add up the recorder, the cameras’ spec-sheet maxima, about 12 percent for the switch, and the router. Multiply by 1.25. Buy a pure sine wave unit whose watt rating covers that with room, then check the vendor’s runtime chart at your load and make sure it’s over ten minutes. Plug the USB cable into the recorder, install NUT, set a shutdown threshold, give Frigate a longer stop grace period, and pull the plug once to prove it works.
That’s an afternoon and a modest purchase, and it closes the last gap in a setup that otherwise doesn’t need anyone else’s servers to keep working.