A UPS for your NAS: sizing and Canadian picks
By Ryan Fournier · Reviewed by Claire Bergeron · Updated A NAS should sit on a UPS, because an unclean power cut during a write can corrupt a pool or a rebuild. For a typical four-bay NAS drawing 40 to 60 W, a 650 to 900 VA line-interactive UPS gives several minutes of runtime — enough for the NAS to see the outage over USB and shut down cleanly. Good Canadian picks are APC Back-UPS and CyberPower units, from roughly CA$110 to CA$200.
Why a NAS needs a UPS
RAID and ZFS protect against a drive failing, not against the power cutting out mid-write. An abrupt outage during a write — or worse, during a parity rebuild — can leave the array inconsistent or, on the wrong filesystem, corrupt. A UPS bridges the gap: it holds the NAS up long enough to flush writes and shut down in an orderly way.
The NAS talks to the UPS over USB. Synology, UGREEN, QNAP, TrueNAS and Unraid all read the UPS status and trigger a safe shutdown when the battery runs low — you set the threshold once.
Sizing: VA and runtime
You are not trying to run the NAS through a long outage — just to survive brief cuts and shut down cleanly on longer ones. For a four-bay NAS at 40 to 60 W, a 650 to 900 VA line-interactive unit gives roughly 5 to 15 minutes, which is ample. A six-bay or a NAS-plus-switch setup wants 900 to 1500 VA.
Choose line-interactive (it regulates minor sags without switching to battery) over a cheap standby unit, and check the unit has a USB port your NAS supports.
Setting up safe shutdown
- Connect the UPS to the NAS by USB.
- In the NAS settings, enable UPS support and set it to shut down when the battery is low (or after N minutes on battery).
- Test it once by pulling the UPS from the wall and confirming the NAS shuts down cleanly before the battery dies.
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Frequently asked questions
What size UPS for a NAS?
For a four-bay NAS at 40–60 W, a 650–900 VA line-interactive UPS gives several minutes — enough to shut down cleanly. Six-bay or NAS-plus-switch setups want 900–1500 VA. You are sizing for a clean shutdown, not to ride out a long outage.
Do I really need a UPS for a NAS?
Strongly recommended. RAID protects against a drive failing, not against power cutting out mid-write, which can corrupt a pool or a rebuild. A UPS plus USB safe-shutdown removes that risk cheaply.
Line-interactive or standby UPS?
Line-interactive. It regulates minor voltage sags without switching to battery, which is easier on the NAS and the battery than a cheap standby unit.
Ryan Fournier covers home-server hardware and efficiency at nasdrives.ca: the right power supply, the UPS, and what a NAS actually draws running around the clock, priced against Canadian hydro rates.
Reviewed by Claire Bergeron, Editor-in-chief