UPS for a NAS - the Cable Matters More Than the Battery

What the NAS actually does when the power goes

Every NAS operating system worth running has the same three-stage response and it is worth knowing before you shop, because it tells you which specification to care about. Stage one: the UPS reports that mains has failed. Stage two: a countdown, either a fixed period you set or a wait until the UPS reports low battery. Stage three: the NAS stops services, unmounts its volumes and powers down cleanly.

The whole sequence is a couple of minutes. It is not a strategy for staying online through an outage, and treating it as one is how people end up buying four times the battery they need. A NAS that shuts down cleanly at minute three has fully succeeded. A NAS with a two-hour battery and no data cable has fully failed, just later.

Synology's implementation lives in Control Panel under Hardware and Power, and their own documentation describes the choice as either a custom delay or waiting until low battery, after which the unit enters Safe Mode. QNAP's QTS exposes the same shape under External Device, with two configurable responses to a power failure: power off after a set period, or enter auto-protection mode, which their documentation describes as stopping all services and unmounting all volumes to protect your data. TrueNAS runs its UPS service on Network UPS Tools and exposes master and slave modes so one machine can watch the UPS and the rest can watch that machine.

The practical consequence: if you already run TrueNAS, or a Linux box, or anything else that can run NUT, then the vendor compatibility list is much less binding than it looks, because NUT talks to a very large number of units directly.

NUT, and why we link it instead of reproducing it

Network UPS Tools is the open-source project that most of this ecosystem quietly rests on. In the project's own words its goal is "to provide support for Power Devices, such as Uninterruptible Power Supplies, Power Distribution Units, Automatic Transfer Switches, Power Supply Units and Solar Controllers", offering a common protocol and set of tools across manufacturers' incompatible ones. It is GPL licensed.

Its hardware compatibility list is the reference document for this question. At the date we read it, it recorded at least 199 manufacturers and 1,436 device models as known compatible - a very different number from any single NAS vendor's tested list.

We link that list rather than reproducing it, and the reason is a rule we apply generally: a stale copy of somebody else's compatibility list is worse than a link to the live one. It is maintained upstream, it changes, and a reader who checks our snapshot from months ago and finds their model missing has been actively misled by us. Go to the source: it is the single most useful page on this subject and it is not ours.

The practical route for most people is simpler than the list suggests. Plug the unit into the NAS by USB, open the UPS settings, and see whether the machine reports a battery. Most modern units present themselves through a generic HID power-device interface and are recognised without any configuration at all. If it appears, you are done. If it does not, the compatibility list is where to look next.

Sizing for a NAS, and the honest gap in it

The sizing method is the one on the hub page and it comes from Synology rather than from us: total the real wattage of everything on the UPS, then divide by the unit's output power factor to get the output rating you need. Synology's worked example puts a 650 W total against a 70% power factor and arrives at 928.5 W.

What we cannot give you is the first number. This site publishes no per-device wattage - not for a four-bay NAS, not for an eight-bay, not for a switch - because we have not fetched and logged a maker datasheet for each machine, and a wattage attributed to a typical device of that shape is not a citation. Your NAS publishes its own figure. On Synology's spec pages it sits under power consumption, usually with two values, one for access and one for hibernation, and a note about the drive configuration it was measured with. That last part matters: the figure moves with how many drives you have fitted, because most of the draw is spinning platters, not the chassis.

Two additions people forget. Drives spinning up simultaneously at power-on draw substantially more than they do at idle, so a unit sized exactly to the running figure has no headroom for a restart. And the network switch between the NAS and everything else needs to be on the same UPS, or the NAS survives the outage in perfect isolation and nobody can reach it.

What our own catalogue can tell you is what is on the shelf at each VA band and what real power those units claim - that table is on the hub, it is a measurement of our inventory, and it is labelled as one rather than dressed up as a recommendation about your hardware.

One UPS, several machines

The common homelab shape is one UPS carrying a NAS, a switch, and one or two other machines, and the question is how the machines that do not hold the USB cable find out about the outage.

All three ecosystems solve it the same way, with different names. Synology's compatibility page states it plainly: any model on their list can extend UPS service to a second Synology product using Network UPS Server, where the first machine connects by USB and relays the information to clients on the local network. QNAP exposes network UPS master and slave modes. TrueNAS calls it master and slave and, being NUT underneath, will happily act as either against a non-TrueNAS peer.

Because they are all speaking the same underlying protocol, mixed environments generally work: a TrueNAS box holding the cable can signal a Linux machine running NUT, and a Synology acting as server can be watched by another Synology. What is not guaranteed is any particular cross-vendor pairing, and we are not going to assert one we have not tested.

Synology adds one hardware caveat worth repeating because it catches people: connect the USB device directly to a USB port on the product. Connecting through a hub is not supported. If your NAS is behind a hub for want of ports, that is the first thing to eliminate when it fails to see the UPS.

Test it, because nobody else will

This is the step that gets skipped and it is the only one that proves the purchase worked. The instruction is not ours - it is on Synology's own compatibility page, attached to the tested list: the UPS power-off function varies by brand and model, so simulate a power outage to ensure the function performs as expected.

Do it deliberately, on a day when nothing is at stake, with nothing important being written. Pull the mains plug on the UPS - not the switch, the plug - and watch. The NAS should log the loss within seconds, count down, and shut down. Then restore power and confirm it comes back up and remounts cleanly.

Repeat it after any change to the chain: a new UPS battery, a firmware update on either device, moving the cable to a different port. And repeat it when the battery is around three years old, because a battery that has aged out will still let the NAS see the UPS and report a healthy state right up until the moment it is asked to actually supply power.

A shutdown chain that has never been tested is a shutdown chain you are guessing about, and the outage is a bad time to find out that the guess was wrong.

Where to go next

The UPS hub has the waveform rule and the sizing method in full · Rackmount units if the NAS lives in a rack · NAS devices and drive compatibility by machine cover the other end of the cable · NAS drives and filling a NAS are the array itself · enclosures share the outlet and want counting into the load.

Frequently asked questions

Sources

Specification claims on this page were checked against the pages below on 4 August 2026. Where a widely-repeated figure did not survive that check, we report what the source says now and say so in the text rather than repeating the familiar number.

1. CyberPower Systems - Active PFC Compatibility (glossary) (cyberpowersystems.com, 2026-08-20)
   Used for: the maker's own definition, quoted verbatim on our pages: Active PFC compatibility is "a feature of UPS systems that delivers sine wave output to prevent unexpected shutdowns or component stress for connected devices with Active PFC power supplies." This is the source for the spec-gate framing - the failure mode named by the maker is an unexpected shutdown, which is precisely the event a UPS was bought to prevent.
2. Synology Inc. - How do I choose a suitable UPS? (Knowledge Center) (kb.synology.com, 2026-08-20)
   Used for: the sizing method used on these pages, quoted rather than paraphrased: "Output power = Total wattage of your devices / Output power factor of a UPS", with Synology's own worked example dividing a 650 W load by a 70% power factor to reach 928.5 W. Also the VA-versus-watts caveat in the vendor's own words: "Different manufacturers have different volt-ampere (VA) ratings based on the performance of their UPS. To determine the total power (W) that the UPS can support, we recommend consulting the manufacturer's product manual." Article last updated 1 April 2024.
3. Synology Inc. - Products Compatibility List - UPS, filtered to tested by Synology (synology.com, 2026-08-20)
   Used for: the full tested list, captured in a rendered browser session: 79 entries, 73 UPS units and 6 management cards or protocol adapters. Also the vendor's own two operational notes, which we quote rather than restate: that any listed model can extend UPS service to a second Synology product over the network, and that "The UPS power-off function varies by brand and model. Please simulate a power outage to ensure the function performs as expected." The list is CATEGORY-level: it does not filter per machine.
4. QNAP Systems - Configuring the UPS settings, QTS 5.2.x (docs.qnap.com, 2026-08-20)
   Used for: that QTS supports three connection modes - USB cable, SNMP over the network, and network UPS slave against another machine acting as the server - and the two configurable responses to a power failure: powering off after a set period, or entering auto-protection mode, which stops services and unmounts volumes.
5. Network UPS Tools project - Network UPS Tools (NUT) - project overview and hardware compatibility list (networkupstools.org, 2026-08-20)
   Used for: the project's own description of its scope - "to provide support for Power Devices, such as Uninterruptible Power Supplies, Power Distribution Units, Automatic Transfer Switches, Power Supply Units and Solar Controllers" - its GPL licence, and the size of its hardware compatibility list at the date read: at least 199 manufacturers and 1,436 device models. We link this list; we do not reproduce it. It is maintained upstream, it changes, and a stale copy of somebody else's compatibility list is worse than a link to the live one.
6. Schneider Electric (APC) - What is the expected life of my APC UPS VRLA battery? (se.com, 2026-08-20)
   Used for: the three-to-five-year expected service life of the sealed lead-acid battery in a typical APC UPS, and that operating temperature is the dominant factor in shortening it. The replacement-cartridge prices quoted on our pages are described as reported figures, not as our measurement - we do not track UPS battery prices.
7. DatacenterDisk - Live catalogue - UPS listings (datacenterdisk.com, 2026-08-20)
   Used for: every price, VA rating, watt rating, outlet count and waveform statement on these pages, read from the seller's own title and feature bullets at seed time and refreshed by the price cron. The waveform coverage rate is recomputed on each render rather than frozen here.