Backup power for Home Assistant: UPS vs power station, sizing and clean shutdown

Also available in French.

A few seconds without mains reboots Home Assistant, drops your router and weakens Zigbee. Longer outages take alarms, leak sensors and notifications offline when you need them most. This guide helps you size backup power, pick UPS vs portable power station, surface battery state in HA and shut down cleanly before the battery dies.

We budget device by device, including Zigbee routers on mains-powered plugs.

Goals

Measure real watts for the smart-home core.
Estimate runtime or required Wh.
Pick gear with suitable transfer time.
Alert on battery and trigger host shutdown.

What must stay on

Device	Why
Home Assistant (Pi 5, Green, etc.)	Automations, history, notify
Zigbee coordinator (USB)	Already counted with the server
Router (+ fibre ONT if separate)	Phone alerts, remote access
Network switch if between router and server	No LAN without it

Battery Zigbee sensors keep working. Mains-powered Zigbee plugs and bulbs are mesh routers—when they lose power, distant sensors may drop off. Check the ZHA or Zigbee2MQTT map before you rely on sensors across the house during an outage.

Measure, don’t guess

Use a plug-in power meter or a smart plug with energy reporting in HA. Label ratings on PSUs are maximums, not typical draw.

Rough references (measure yours):

Device	Reference
Home Assistant Green	~1.7 W idle, ~3 W load (official spec)
Raspberry Pi 5	27 W PSU rating—actual HA load much lower; measure
ISP router	Often the largest load—read brick label and measure

Autonomy math

P = sum of average watts (W).
E = usable Wh ≈ battery Wh × usable fraction × inverter efficiency.
Runtime (h) = E / P.
Capacity needed for t hours: C ≈ P × t / (usable × efficiency).

Example (illustrative only—replace with your measurements): Green 3 W + router 15 W + switch 2 W → P = 20 W. A 300 Wh station at ~90 % usable and ~85 % AC efficiency → E ≈ 230 Wh → ~11 h. For 24 h at 20 W AC out you might need ~630 Wh before margins.

The router often dominates the Pi. Unplug TVs and chargers from the backed-up strip.

DC tip: Green accepts 12 V DC (official spec)—a station with a suitable 12 V port can skip one conversion step; verify voltage, current and connector before wiring.

UPS vs power station

	UPS	Power station
Role	Minutes to a few hours + clean shutdown	Larger reserve, sometimes portable
Transfer	Designed for fast switchover	Only if UPS/EPS mode—read spec
HA integration	Often USB HID + NUT	Rarely standard; app/cloud
Typical 20 W runtime	~1–3 h class UPS	Half-day to multi-day class

Pass-through charging ≠ UPS. Confirm transfer time on the datasheet.

Permanent mains charging of a station 24/7: read manufacturer guidance on battery longevity.

Shopping: UPS with USB and NUT compatibility—Amazon UK UPS search · US.

Test a real outage

Charge fully, plug only HA + router + switch, unplug the UPS/station from the wall (not the device power button), confirm no reboot, Zigbee still responds, note % drop over 30 min and recalculate runtime.

NUT in Home Assistant (UPS path)

USB from UPS to HA host.
Install Network UPS Tools add-on; configure driver (often usbhid-ups).
Add NUT integration (docs).
Entities like sensor.ups_battery_charge, sensor.ups_status.

Notify on battery:

alias: Mains lost - on battery
triggers:
  - trigger: state
    entity_id: sensor.ups_status
    to: "On Battery, Battery Discharging"
actions:
  - action: notify.mobile_app_your_phone
    data:
      message: "Power cut: smart home on battery."

Shutdown below 25 % while still on battery:

alias: Low battery - shutdown HA
triggers:
  - trigger: numeric_state
    entity_id: sensor.ups_battery_charge
    below: 25
conditions:
  - condition: state
    entity_id: sensor.ups_status
    state: "On Battery, Battery Discharging"
actions:
  - action: notify.mobile_app_your_phone
    data:
      message: "Battery under 25%: shutting down in 2 minutes."
  - delay: "00:02:00"
  - action: hassio.host_shutdown

Power stations without NUT

Use a witness device on non-backed-up mains—e.g. ESP32 on a normal USB charger going unavailable in HA when grid drops (ESPHome sensor). Avoid cloud-only battery apps during internet loss unless the router is also on backup.

After power returns

After hassio.host_shutdown, Pi/Green may need a power cycle on the backup outlet if the UPS never cut output. Plan a “Home Assistant started” notification.

Common mistakes

Sizing from PSU labels.
Forgetting router/ONT.
Treating pass-through as UPS.
Loading heaters or printers on the backup strip.
Shutdown automation without “on battery” condition.

Also: Install Home Assistant · CO₂ on Pi · solar outdoor camera.

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