Safe Shutdown for TrueNAS Scale and Proxmox using an APC Back-UPS and NUT Remote Shutdown

Many small businesses and homelabs today operate multiple systems at the same time:
a TrueNAS Scale storage server, a Proxmox virtualization host and several
network devices such as routers or switches.

Even a short power outage can cause unclean shutdowns of virtual machines,
ZFS pools or running services. In this article I demonstrate a simple and
reliable setup where TrueNAS Scale acts as the central UPS master and safely
shuts down a Proxmox host over the network.

Goal of this setup

• Connect the UPS directly to TrueNAS Scale via USB
• Use Proxmox as a remote monitoring client
• Automatically shut systems down during power outages
• Protect VMs and ZFS pools from sudden power loss
• Keep routers and network devices online as long as possible

Hardware used

• APC Back-UPS RS 550G
• TrueNAS Scale
• Proxmox VE
• Standard local network

Architecture

UPS (USB)
   │
   └── TrueNAS Scale (NUT Master)
            │
            └── Network
                    │
                    └── Proxmox VE (NUT Client)
  

Configure TrueNAS Scale

The UPS is connected directly to the TrueNAS system via USB.
TrueNAS then operates as the central NUT server.

Enable the UPS service

• System Settings → Services → UPS
• UPS Mode: Master
• Driver: APC ups 2 Back-UPS Pro USB
• Port: automatically detected

Recommended settings

Shutdown Mode: UPS goes on battery
Shutdown Timer: 30
Host Sync: 15
Power Off UPS: disabled
  

Enable remote monitoring

Important: enable “Remote Monitor” so that other systems
can access the UPS status over the network.

Optional runtime override

Especially with APC Back-UPS devices, runtime estimation can be inaccurate.
A conservative override is therefore recommended.

override.battery.runtime.low = 60
  

Check UPS status

upsc ups@localhost
  

Important values:

battery.runtime
battery.runtime.low
battery.charge
ups.status
  

Configure Proxmox as NUT client

Install NUT

apt update
apt install nut-client
  

/etc/nut/nut.conf

MODE=netclient
  

/etc/nut/upsmon.conf

MONITOR ups@192.168.178.100 1 upsmon fixmepass slave
MINSUPPLIES 1
SHUTDOWNCMD "/sbin/shutdown -h now"
POWERDOWNFLAG /etc/killpower
  

Where:

• 192.168.178.100 = IP address of the TrueNAS server
• ups = UPS name
• upsmon/fixmepass = monitoring user configured in TrueNAS

Enable the service

systemctl enable nut-client
systemctl restart nut-client
  

Test the connection

Port test

nc -zv 192.168.178.100 3493
  

Read UPS status

upsc ups@192.168.178.100
  

Monitor logs

journalctl -f | grep -Ei "ups|nut|shutdown|fsd"
  

Real-world test

After disconnecting the UPS from mains power,
the following sequence should appear:

UPS on battery
→ Shutdown timer starts
→ FSD (Forced Shutdown)
→ Proxmox shuts down
→ TrueNAS shuts down
  

Important findings

• Low-cost APC Back-UPS devices often provide inaccurate runtime estimations
• After replacing the battery, the runtime curve may require recalibration
• A conservative shutdown timer is usually more reliable than percentage indicators
• 30 seconds is sufficient for many small environments

Conclusion

For small server environments this setup is surprisingly effective.
Even affordable APC Back-UPS models are capable of safely shutting down
TrueNAS and Proxmox systems while protecting data integrity.

Users who later require advanced monitoring, accurate battery analytics
or SNMP integration can easily upgrade to an APC Smart-UPS system.

For homelabs, small offices and private virtualization environments,
the combination of TrueNAS Scale, Proxmox and NUT already provides
a highly reliable and professional solution.