Battery Backup for a NAS

Buy the data cable, not the runtime. The valuable thing a UPS does for a NAS is tell it that power has failed so it can shut down cleanly. Battery minutes are only there to make that possible — sizing for a long runtime is the usual way people overspend on this.

A note on this guide: this rests on how UPS signalling and NAS shutdown behaviour are specified, not on outages staged here. See how we test.

The problem, as it appears

The NAS has been running for a year without incident. The power flickers one evening, everything comes back, and the array reports that it is resyncing — or worse, it comes back and says nothing while a file written that afternoon is not what it should be.

Nobody was in the room. That is the whole difference between this and a desktop.

Why the desktop version of the argument disappoints

Because on a desktop the cost of an outage is visible and immediate: you lose what you had not saved, you notice instantly, and you redo it.

A NAS is doing work you are not watching. Scheduled backups, media indexing, an array scrub, a write from another machine. An interruption partway through any of those lands on stored data rather than on a live session, and the failure surfaces later — sometimes much later, when you go looking for the file.

So the reason to protect it is not continuity of service. It is that the thing being interrupted is the copy you were relying on.

What actually matters

The signal, not the battery

This is the specification to buy on, and it is easy to miss because it is not the headline figure.

A UPS with a USB or network data connection can tell the NAS that it is running on battery. The NAS sees that, waits a set period in case power returns, and then shuts itself down in an orderly way with plenty of charge left.

Without that connection you have bought a delay. The NAS runs happily until the battery empties and then stops just as abruptly as it would have done, only later.

Check two things: that the UPS exposes a data connection your NAS supports, and that your NAS software lists UPS support at all. Most mainstream units do; it is worth confirming rather than assuming.

Size for shutdown, not for uptime

Once the goal is a clean shutdown, the runtime requirement collapses. You need enough to notice the outage, wait out a brief flicker, finish in-flight writes and power down — minutes, not hours.

That is a much smaller and cheaper unit than people buy. Sizing for continued service through a long outage is a different requirement and it should be a deliberate choice rather than a default.

Measuring what the NAS actually draws is the honest way to size it, and the same meter used for how much power a gaming PC uses does the job.

Put the network on it as well

If the NAS is going to signal, log or complete a transfer during those minutes, the switch and router being dead is unhelpful. Both draw very little.

For a modest amount of extra load you keep a working network for the same window, which also means the NAS can still tell you what happened.

A UPS is not a backup, and this is where people conflate them

Worth stating because a NAS invites the confusion. Redundant drives plus a UPS feels like comprehensive protection and covers exactly two failure modes: a drive dying, and a bad shutdown.

It does nothing about deletion, corruption, ransomware or the box itself failing — which is the argument set out in RAID is not a backup. The NAS is the thing that needs backing up, not the backup.

The battery is a consumable and nobody is watching it

UPS batteries last a few years and then hold progressively less charge. On a desktop you notice, because the machine dies during an outage while you are sitting at it.

On a NAS, a dead UPS battery is silent until the one evening it matters. Whatever schedule you keep, this is the machine where a calendar reminder earns its place — when to replace a UPS battery covers the signs.

The options, and who each suits

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A UPS with a data connection suits every NAS, and the connection is the reason to buy rather than a feature to compare later.

A named unit with verified specifications, if you would rather not choose blind — the APC Back-UPS Pro BX1500M, reviewed.

The NAS itself is the thing being protected, and it is worth remembering it still needs a backup target that is not inside it.

An outlet tester suits ruling out the socket and its ground before assuming the equipment is at fault, and it is the cheapest instrument here.

The mistake to avoid

Buying runtime instead of signalling. The battery figure is the number on the box and the data connection is the feature that does the work, so it is entirely possible to spend more and end up with a NAS that still shuts down uncleanly — just twenty minutes later, and with more confidence that it was protected.

The second mistake is putting the NAS on a surge-only outlet. Most units divide their outlets into battery-backed and surge-only, the labelling is small, and a NAS on the wrong side of that divide gets no shutdown warning at all. Check the socket, not just the unit.