Precision for Entities in Home Assistant

Home Assistant has had the ability to set the display precision of sensor value for a while. If you click on the sensor entity (in the Settings > Devices & Services > Entities) and then the little cogwheel for the entity's settings, you'll see something like this:

Except if you don't…

The entity in question here is the charge of my home battery in percent. This is ingested via the MQTT integration with the following configuration:

mqtt:    
  sensor:                     
    - unique_id: "battery_charge"  
      name: "Battery Charge"                              
      state_topic: "N/[...]/system/0/Dc/Battery/Soc"
      value_template: "{{ value_json.value }}"

Problem now was, that I couldn't set the display precision because the drop-down simply wasn't available. Took me a while to figure out: apparently the UI only allows you to configure the precision, if the sensor has a unit_of_measurement value like this:

mqtt:    
  sensor:                     
    - unique_id: "battery_charge"  
      name: "Battery Charge"                              
      state_topic: "N/[...]/system/0/Dc/Battery/Soc"
      value_template: "{{ value_json.value }}"
      unit_of_measurement: "%"

Suddenly the precision setting in the UI becomes available and my cards no longer display things like "95.4562334" instead of the more reasonable "95.5 %".