Creating a Differential Sensor In Home Assistant

If you've fiddled enough in Home Assistant over time, you might have encountered the situation in which it is useful to trigger off of, or simply display, a calculated value. There are a few different ways this can be accomplished but this post focuses on creating a specific sensor to do both: be used as a trigger value for automation, and also displayed neatly in a dashboard.

The Basic Need

I have all sorts of temperature and humidity sensors throughout the house and outside the house. I personally like the simplicity, size, and battery duration of the Aqara zigbee sensors, but they must be in range of the zigbee network (or HA USB dongle) to behave. I also have a Tempest weather station which exposes its sensors via its API through a HACS integration, which is useful for the weather data and accurate outside temperature.

But I also have a couple of outbuildings where it's useful to monitor the interior temperature. In those cases, they're too far away from the interior zigbee network to behave, but I have WiFi access, so I use the little Shelly H&T original WiFi-enabled sensors. They work great, give me accurate data, and the battery performance is … okay. They tend to eat batteries in the deep of winter, averaging one per month, but it's a price I'm willing to pay at this point for the data.

It's very useful to see at a glance the difference in temperature, so to do that I create … another sensor!

Time for a Template

As I've done before, I jump into my /config/mz-templates.yaml file to add a simple sensor:

  - sensor:
      - name: "Temp 1 and Temp 2 Difference"
        unique_id: "sensor.temp1_temp2_diff"
        unit_of_measurement: "°F"
        state: >
          {% set first = states('sensor.temp1_temperature') | float %}
          {% set second = states('sensor.temp2_temperature') | float %}
          {%- if is_number(first) and is_number(second) %}
            {%- set tempdiff = (second - first) | float | round(2) | abs %}
            {{ tempdiff }}
          {%- else %}
            N/A
          {%- endif %}
        state_class: measurement
        device_class: temperature

For example, the temp1 and temp2 sensors would show up like this in the dashboard:

The state: declaration in the yaml configuration does a couple of simple things:

1. Obtains the temperature values (as floating points) for the two sensors in question;
2. Does a quick "sanity check" to validate they are both numeric in value (or outputs N/A otherwise); and
3. Calculates the absolute value difference between the data points.

The first time I set up the diff I chose to go without absolute value as I expected the temp2/second sensor to have a value greater than temp1/first. I later added the abs conversion to make the needle gauge behave better. Your mileage and needs will vary, especially if you're using a specific range/value to trigger or conditionally execute automations. After the sensor is configured I can add the  sensor.temp1_temp2_diff sensor to a needle gauge and my dashboard feature is complete:

Simple and Effective

This was a useful addition to one of my HA dashboards to visually identify if supplemental heating is necessary to turn on or, during the non-winter months, if it's useful to open some windows.

Little things like these sensors really add a level of power to what can be done in Home Assistant. Good luck!