A UPS (Uninterruptible Power Supply) protects servers and workstations from sudden power loss, but hardware alone is not enough. Without software monitoring the UPS, your system will still crash hard when the battery runs out. Network UPS Tools (NUT) is the standard open-source solution: it speaks to hundreds of UPS models, monitors battery state, and triggers a clean shutdown before the battery dies. This guide walks through a single-machine NUT setup — one host talking directly to its own UPS — the most common scenario for a home lab or small server.

Prerequisites and Terminology

NUT has three moving parts you need to understand before touching config files:

• upsd — the daemon that reads UPS data and serves it over a socket
• upsmon — monitors UPS state and triggers shutdown commands
• driver — the per-model binary that actually talks to the UPS hardware (USB, serial, SNMP, etc.)

For a single machine, all three run on the same host. The UPS connects via USB (most common for consumer and prosumer units) or a serial port.

Install NUT

Debian / Ubuntu

sudo apt update
sudo apt install nut nut-client nut-server

Fedora / RHEL 9 / Rocky Linux

sudo dnf install nut

On RHEL-family systems the package ships both server and client components together.

Arch Linux

sudo pacman -S nut

After installation NUT's config files live in /etc/nut/ on all distributions.

Detect Your UPS

Plug in the UPS via USB, then run the scanner. It probes USB and other buses and prints the driver block you need:

sudo nut-scanner -U

Output will look roughly like this (yours will differ):

[nutdev1]
    driver = "usbhid-ups"
    port   = "auto"
    vendorid = "051d"
    productid = "0002"
    product  = "Back-UPS ES 700"
    vendor   = "American Power Conversion"

Copy this block — you will paste most of it into ups.conf. If nut-scanner finds nothing, check that the USB cable is data-capable (not charge-only) and that you ran the command with sudo.

Configure the Driver — ups.conf

Edit /etc/nut/ups.conf. Replace any placeholder content with your detected block, giving the UPS a short name (here, myups):

sudo nano /etc/nut/ups.conf

[myups]
    driver = usbhid-ups
    port   = auto
    desc   = "APC Back-UPS ES 700"

If nut-scanner returned vendorid and productid, add them too — they prevent NUT from grabbing the wrong USB device if you ever connect multiple UPS units.

[myups]
    driver    = usbhid-ups
    port      = auto
    vendorid  = 051d
    productid = 0002
    desc      = "APC Back-UPS ES 700"

Common drivers: usbhid-ups covers most USB HID-compliant units (APC, Eaton, CyberPower); blazer_usb covers many cheaper USB units from Mecer, Voltronic, and rebrands; nutdrv_qx covers Q-series protocol units. The NUT hardware compatibility list at networkupstools.org maps models to drivers.

Set the Operating Mode — nut.conf

This single file tells NUT what role this machine plays:

sudo nano /etc/nut/nut.conf

MODE=standalone

standalone means one machine, one UPS, no network slaves. Use netserver only if other machines will monitor this UPS over the network.

Configure the Server — upsd.conf and upsd.users

upsd.conf controls what interfaces upsd listens on. For a standalone setup, localhost is sufficient:

sudo nano /etc/nut/upsd.conf

LISTEN 127.0.0.1 3493
LISTEN ::1 3493

Then define at least one user in upsd.users. You need an admin-type user for upsmon to authenticate with:

sudo nano /etc/nut/upsd.users

[upsmon_local]
    password  = changeme_strong_password
    upsmon master

Pick a real password. Even though this only listens on localhost, it is still a credential stored in a plain-text file — restrict its permissions:

sudo chmod 640 /etc/nut/upsd.users
sudo chown root:nut /etc/nut/upsd.users

Configure the Monitor — upsmon.conf

upsmon is what actually calls shutdown when the battery hits a critical level:

sudo nano /etc/nut/upsmon.conf

The critical line is MONITOR. Its format is upsname@host powervalue username password type:

MONITOR myups@localhost 1 upsmon_local changeme_strong_password master

MINSUPPLIES 1
SHUTDOWNCMD "/sbin/shutdown -h +0"
NOTIFYFLAG ONLINE  SYSLOG+WALL
NOTIFYFLAG ONBATT  SYSLOG+WALL
NOTIFYFLAG LOWBATT SYSLOG+WALL
NOTIFYFLAG COMMOK  SYSLOG
NOTIFYFLAG COMMBAD SYSLOG
POLLFREQ 5
POLLFREQALERT 5
HOSTSYNC 15
DEADTIME 25
RBWARNTIME 43200
NOCOMMWARNTIME 300
FINALDELAY 5

SHUTDOWNCMD is what runs when the battery is critically low. /sbin/shutdown -h +0 halts immediately. Adjust to shutdown -h +2 if you want a two-minute grace window. The powervalue 1 means this host needs 1 power supply from this UPS to stay alive — correct for a standard desktop or server.

Start and Enable the Services

Debian / Ubuntu / Arch

sudo systemctl enable --now nut-server nut-monitor

On Debian/Ubuntu you may also need:

sudo systemctl enable --now nut-driver@myups

Fedora / RHEL / Rocky

sudo systemctl enable --now ups nut-server nut-monitor

The RHEL family uses a ups target that groups the driver and server. If the service name differs on your release, use systemctl list-units 'nut*' to see what is available.

Verify Everything is Working

Query live UPS data with upsc:

upsc myups@localhost

You should see a wall of variables — the important ones:

• ups.status: OL — Online (on mains power)
• battery.charge: 100 — Battery fully charged
• battery.runtime: 1800 — Estimated runtime in seconds

OB means on battery, LB means low battery (shutdown imminent). Check that upsmon is connected:

sudo upsc myups@localhost ups.status
sudo systemctl status nut-monitor

To test driver communication without triggering a real shutdown, briefly unplug the UPS mains cable and watch the status flip to OB in the output of a looping upsc:

watch -n 2 upsc myups@localhost ups.status

Troubleshooting

Driver fails to start — permission denied on USB device

The nut user needs access to the USB device node. On most distros the NUT package ships a udev rule at /lib/udev/rules.d/62-nut-usbups.rules that handles this. If it is missing, reload udev and replug the UPS:

sudo udevadm control --reload-rules
sudo udevadm trigger

upsc returns "Error: Driver not connected"

The driver has not started or crashed. Check its journal:

sudo journalctl -u nut-driver@myups -n 50

Common causes: wrong driver = value in ups.conf, typo in the UPS name, or a USB permission issue.

upsmon reports "COMMBAD" then does nothing

If upsd is not reachable, upsmon logs COMMBAD but will not shut down until DEADTIME seconds have elapsed. Check that nut-server is running and that the credentials in upsmon.conf match upsd.users exactly — including case.

System shuts down immediately after config

Check that MINSUPPLIES is set to 1 and that upsc myups@localhost ups.status shows OL before starting services. If NUT cannot reach the UPS at startup it may treat it as a lost supply and trigger shutdown.