Setting Up NUT (Network UPS Tools) for Automated Safe Shutdown on Power Loss

Buying a UPS only solves half the problem. Left alone, it just runs on battery until it dies — and if that happens while your NAS or hypervisor is still writing data, you get the exact filesystem corruption a UPS was supposed to prevent. NUT (Network UPS Tools) is the free, open-source software that closes that gap: it watches the UPS over USB and triggers an orderly shutdown of your host(s) before the battery runs out.

Search intent

Someone searching this already owns a UPS and wants it to actually protect their systems — a config walkthrough for wiring a UPS into an automated shutdown, not a UPS buying guide.

What you’ll need

A UPS with a USB connection to a Linux host (most consumer/prosumer UPS units — APC, CyberPower, Eaton — support USB-based monitoring; check your model’s manual for USB HID support if unsure).
A Linux machine to run NUT — this can be the same host doing the work (a NAS, hypervisor box) or a dedicated small always-on machine.
Root/sudo access, 20-30 minutes.

How NUT is structured

NUT has three components, and understanding them upfront makes the config make sense:

A driver (usbhid-ups for most modern USB UPS units) — talks directly to the UPS hardware and translates its status into a common format.
upsd — the server that reads from the driver and serves UPS status to clients over the network (even if that’s just localhost).
upsmon — the monitoring client that watches upsd, decides when the battery is critically low, and triggers the actual shutdown command.

For a single-host setup (NUT running on the same machine the UPS is plugged into and protecting), all three run locally. For a shared setup (one UPS, multiple hosts), other machines run just upsmon pointed at the host serving upsd over the network.

Step 1: Install NUT

On Debian/Ubuntu:

sudo apt update
sudo apt install nut

This installs the driver, upsd, and upsmon together. (Some distros split nut-server and nut-client into separate packages — install both if so.)

Step 2: Identify your UPS and configure the driver

Plug the UPS into the host via USB, then check it’s detected:

lsusb

You should see your UPS vendor listed (APC, CyberPower, Eaton, etc.). Then edit /etc/nut/ups.conf to add your UPS:

[myups]
    driver = usbhid-ups
    port = auto
    desc = "Main server rack UPS"

usbhid-ups handles the vast majority of modern USB UPS units regardless of brand, since USB HID power-device class is a standard. If your specific model needs a different driver, NUT’s hardware compatibility list (bundled in the docs) will name it.

Step 3: Configure `upsd` to listen and set access mode

Set NUT’s operating mode in /etc/nut/nut.conf:

MODE=standalone

(Use netserver instead if other hosts will monitor this same UPS over the network.)

Then add a monitoring user in /etc/nut/upsd.users:

[upsmon_local]
    password = <choose-a-strong-password>
    upsmon primary

Lock down the permissions on this file, since it holds a plaintext password:

sudo chmod 640 /etc/nut/upsd.users

Step 4: Configure `upsmon`

Edit /etc/nut/upsmon.conf and add:

MONITOR myups@localhost 1 upsmon_local <same-password-as-above> primary
MINSUPPLIES 1
SHUTDOWNCMD "/sbin/shutdown -h +0"

MONITOR ... primary means this host owns the UPS connection directly. The 1 is the number of power supplies this UPS represents to the host (leave at 1 for a typical single-UPS setup). SHUTDOWNCMD is what actually runs once upsmon decides the battery is critically low — the default triggers an immediate clean shutdown.

Step 5: Start the services

sudo systemctl enable --now nut-server
sudo systemctl enable --now nut-client

(Service unit names vary slightly by distro — nut.target, nut-server, and nut-monitor are the common variants; check systemctl list-units | grep nut if the names above don’t match.)

Step 6: Verify it’s actually working

Query live UPS status:

upsc myups@localhost

This dumps every variable NUT can see — battery charge, load, status, runtime estimate. Two worth watching specifically:

upsc myups@localhost battery.charge
upsc myups@localhost ups.status

ups.status should read OL (on line power) normally, and switch to OB (on battery) when you unplug the UPS from the wall — test this deliberately to confirm NUT notices the transition before trusting it in a real outage.

Step 7: Test the shutdown trigger (during a maintenance window only)

Don’t wait for a real outage to find out SHUTDOWNCMD doesn’t work. Force NUT into its shutdown path:

sudo upsmon -c fsd

Only run this against a host you’re prepared to have shut down right now — it triggers the real configured shutdown command, not a dry run. Confirm the host actually powers down cleanly, then bring it back up and verify NUT reconnects to the UPS on boot.

Multi-host setups: shutting down more than one machine from one UPS

If one UPS powers a NAS and a hypervisor host, run upsd (mode netserver) on whichever machine has the USB connection, then on the other host(s) configure upsmon as a secondary:

MONITOR myups@<primary-host-ip> 1 upsmon_remote <password> secondary

Add a matching user in the primary’s /etc/nut/upsd.users, and open the NUT port (3493/tcp) between the hosts on your local network only — this port has no reason to be reachable from outside your LAN. Now both hosts shut down together when the shared UPS hits its threshold, rather than only the one it’s physically plugged into.

Platform notes

Proxmox: no built-in UPS UI, but NUT installs and runs as a normal Debian package on the Proxmox host itself, and SHUTDOWNCMD can be scripted to gracefully stop guests (via pvesh or qm shutdown) before the host powers off.
TrueNAS: has NUT integration built into the web UI (Services → UPS) — point it at the USB-connected UPS and set a shutdown threshold there instead of hand-editing config files.
Synology/QNAP: use their own native UPS monitoring (DSM/QTS Control Panel → Hardware → UPS) rather than NUT — see the best UPS for a home server rack guide’s shutdown-integration section for that path.

Bottom line

NUT is free, well-documented, and the standard answer for Linux-based UPS shutdown automation outside the Synology/QNAP walled gardens. The setup is a handful of config files, but the step people actually skip is Step 7 — testing the real shutdown trigger during a planned window. Do that test before you need it to work during an actual outage.