A UPS gives a homelab time to ride through a short outage or begin an orderly shutdown. It reduces the risk created by abrupt power loss, but it cannot guarantee filesystem integrity, application consistency, battery runtime, or recovery from every hardware and software failure.
Editorial basis: This guide follows current TrueNAS and Network UPS Tools documentation plus published device specifications. It does not claim personal runtime tests, ownership, or hands-on product experience.
On this page
- Measure load and define the shutdown objective
- Waveform and power-supply compatibility
- Product checks before purchase
- Configure the current TrueNAS UPS service
- Primary and secondary shutdown order
- Test without assuming the outcome
- Frequently Asked Questions
- Does a UPS guarantee that ZFS or applications cannot be corrupted?
- What is the NUT shutdown order for one primary and several secondaries?
- How much runtime will a UPS provide?
- Where is UPS configuration in current TrueNAS SCALE?
Measure load and define the shutdown objective#
Measure the wall-power draw of every device that must remain powered long enough to shut down: storage server, network switch, router, and any secondary systems that depend on the same UPS. Use the UPS manufacturer’s runtime curve for the measured load and allow margin for battery aging and shutdown variability.
VA and watts are different limits. The protected load must remain below both ratings. A fixed multiplier or advertised no-load runtime is not a substitute for the vendor runtime curve and an actual shutdown test.
Waveform and power-supply compatibility#
Some active-PFC power supplies are sensitive to stepped approximations while others operate normally. Pure-sine-wave output can reduce compatibility uncertainty, but it is not a blanket guarantee of PSU compatibility or data protection. Check the server and UPS manufacturers’ guidance for the exact models.
Product checks before purchase#
The APC BR1500MS2 and similar line-interactive units are examples in this buying path. Before ordering, verify:
- continuous watt and VA limits at the intended load;
- runtime curve at the measured load;
- output waveform and PSU compatibility;
- which outlets are battery-backed rather than surge-only;
- a USB, serial, or network data interface supported by the selected NUT driver;
- battery replacement availability and warranty terms.
A plug-in power meter can help establish the load before sizing the UPS. It does not simulate startup surges or prove runtime.
Configure the current TrueNAS UPS service#
Use the documentation for the installed TrueNAS release. The current official flow is System > Services, locate UPS, then select Edit. TrueNAS documents the available fields on its UPS Services Screen and the setup flow in Configuring UPS Service.
For a directly connected USB UPS, choose the TrueNAS Master mode, select the driver for the exact device, and select the detected port or auto when supported. To identify a USB device in the current UI, TrueNAS directs administrators to System > General Settings > GUI > Settings, enable Show Console Messages, then inspect the message generated when the device is connected.
For another machine powered by the same UPS, choose the TrueNAS Slave mode on that secondary system and configure the primary host and port. Restrict NUT monitoring to trusted networks and replace default credentials. TrueNAS warns that auxiliary parameters are unsupported and can cause undefined behavior, including data loss, so use them only when required by version-specific documentation.
Primary and secondary shutdown order#
NUT uses the terms primary and secondary; the current TrueNAS UI labels the corresponding modes Master and Slave. During a critical event, the primary marks the UPS for forced shutdown. Secondary systems see that state, run their local shutdown commands, and disconnect. The primary waits for secondaries to disconnect, up to the configured Host Sync interval, before starting its own shutdown. If Power off UPS is enabled and the device and driver support it, the UPS can be commanded off late in the sequence.
This ordering is documented by the NUT upsmon manual. Host Sync is a timeout, not proof that every secondary completed shutdown. Size and test the sequence so the battery can support the entire process.
Test without assuming the outcome#
- Confirm the UPS status is online and that charge, load, and runtime fields are plausible.
- Schedule a maintenance window with current backups and console access.
- Remove utility input to the UPS, not power from the protected load, and confirm the status changes to on-battery.
- Verify each secondary receives the event and completes shutdown before the primary.
- Verify the primary shutdown and optional UPS power-off behavior for the exact driver and hardware.
- Restore utility power and confirm the intended restart behavior.
Repeat the test after material load, firmware, battery, network, or shutdown-policy changes. Battery self-tests and replacement intervals are model-specific; a displayed charge percentage alone does not establish usable runtime or battery health.
Frequently Asked Questions#
Does a UPS guarantee that ZFS or applications cannot be corrupted?#
No. A UPS can provide time for an orderly shutdown and reduce exposure to abrupt power loss. Filesystem, application, controller, cabling, battery, configuration, and hardware failures still require backups and tested recovery.
What is the NUT shutdown order for one primary and several secondaries?#
The primary signals a forced shutdown. Secondaries initiate their local shutdown and disconnect first. The primary waits up to Host Sync, then shuts down itself. A supported UPS can be powered off afterward when that option is enabled.
How much runtime will a UPS provide?#
Runtime depends on measured load, battery condition, temperature, inverter behavior, and device overhead. Use the exact manufacturer’s runtime curve and verify the configured shutdown sequence under controlled conditions.
Where is UPS configuration in current TrueNAS SCALE?#
Open System > Services, locate UPS, and select Edit. Use the TrueNAS documentation version selector to match the installed release.