MQTT sensor from MQTT

Configuration
9

Sorry, that’s my mistake. On closer inspection of your MQTT Explorer screenshot, I see that the key word is payload not Payload.

mqtt:
  sensor:
    - name: Heat Pump Power
      unique_id: hp_pwr_abc123
      state_topic: Heatpump
      value_template: "{{ value_json.payload.Watt }}"
      device_class: power
      unit_of_measurement: W
      json_attributes_topic: Heatpump
      json_attributes_template: "{{ value_json.payload | tojson }}"

Simply change it in your MQTT Sensor’s configuration then execute Developer Tools → YAML → Reload Manually Configured MQTT Entities.