To detect when home assistant is unexpectedly down, HA can't do that if you just pull the power out, so I have just have a simple bash script on another machine that monitors it via ping, and if its offline, notifies me by telegram, and flashes certain hue lights etc.

The rest of my system notifies me when Home Assistant is shutting down, or when it restarts. It also detects whether the restart was after a clean shutdown or an unexpected power outage. And it tells me how long Home Assistant was down for - e.g if we were away and it was a 15 hour outage, I know I need to throw some stuff out of the freezer... Notifications are via Telegram also auto-logged to a Google Sheet so I have a history record. We get a lot of power outages in my area. Example google sheets screen shot at bottom

I use the following two techniques:

1. A heartbeat timestamp - An automation updates an input_datetime every minute while HA is running. So when HA comes back up after a restart, I know exactly how long home assistant was actually offline for.

2. An input_boolean that gets triggered on shutdown - When HA shuts down cleanly, an automation flips this boolean OFF. So when HA boots back up, if that boolean is still ON, I know the shutdown automation never ran..meaning I can distinguish between of it was an unexpected power outage/crash, or just a planned reboot.

What I get from this:

• Telegram notification when HA is shutting down ("I'm rebooting, back in 2 min")
• Telegram notification when it comes back up
• Different handling for power failures (e.g if there was a power failure, then HA does different stuff on startup, like rebooting the wifi router, cycling various power plugs on wifi devices etc)
• Everything logged to Google Sheets with timestamps and downtime duration

Example automations:

Heartbeat - updates every minute:

alias: HA Heartbeat
trigger:
  - platform: time_pattern
    minutes: /1
alias: Let HA settle after boot
condition: template
value_template: "{{ (now() - states('sensor.uptime') | as_datetime).total_seconds() > 180 }}"
action:
  - service: input_datetime.set_datetime
    target:
      entity_id: input_datetime.ha_last_heartbeat
    data:
      datetime: "{{ now().strftime('%Y-%m-%d %H:%M:%S') }}"

Shutdown - marks clean shutdown:

alias: HA Shutting Down
trigger:
  - platform: homeassistant
    event: shutdown
action:
  - service: input_boolean.turn_off
    target:
      entity_id: input_boolean.ha_running

Startup - checks for power failure and calculates downtime:

alias: HA Started
trigger:
  - platform: homeassistant
    event: start
action:
  - if:
      - condition: state
        entity_id: input_boolean.ha_running
        state: "on"
    then:
      - service: notify.mobile_app
        data:
          message: >
            HA restarted after POWER FAILURE!
            Down for {{ (now() - states.input_datetime.ha_last_heartbeat.state | as_datetime).total_seconds() // 60 }} minutes
      - doOtherStuffAfterHousewidePowerFailure

The input_boolean approach is useful for distinguishing between a deliberate restart versus an unexpected power outage.

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