Solar panel pump with differential in/out temperature sensing for pump control

solar.yaml

input_number:
  ### Solar Pump Hysteresis Offsets ##
  pump_off_offset:
      name: Pump OFF offset
      initial: 0.1
      min: 0
      max: 2
      step: 0.1

  pump_on_offset:
     name: Pump ON offset
     initial: 3
     min: 0
     max: 10
     step: 0.5

sensor:
  ## Water Out Temp ##
  - platform: mqtt
    state_topic: "/solar/exit/temperature"
    name: "Solar Temperature"
    qos: 0
    unit_of_measurement: "C"

  ## Water In Temp ##
  - platform: mqtt
    state_topic: "/solar/Tin/temperature"
    name: "Solar Input Temperature"
    qos: 0
    unit_of_measurement: "C"

  # GPIO 12 for PWM Control
  - platform: mqtt
    state_topic: "/solar/pwm/12"
    name: "Solar Motor Value"
    qos: 0

  - platform: template
    sensors:
        temp_diff:
          friendly_name: Temperature Difference
          value_template: >
            {% if ((states.sensor.solar_temperature.state | float ) - (states.sensor.pool_temperature.state | float))
              < (states.input_number.pump_off_offset.state | float ) %} pump_off
            {% elif ((states.sensor.solar_temperature.state | float ) - (states.sensor.pool_temperature.state | float))
              > (states.input_number.pump_on_offset.state | float ) %} pump_on
            {% else %} pending {% endif %}

switch:
  - platform: mqtt
    name: "Solar Pump"
    state_topic: "/solar/pwm/12"
    command_topic: "/solar/pwm/12"
    payload_on: "1023"
    payload_off: "0"
    qos: 1
    retain: true

Thanks for your prompt response really appreciate your help.
As I am a novice with Home assistant, do I just add this to my configuration.yaml?

That's a bit more difficult to explain. I've got another sensor in there sensor.pool_temperature that is part of the differential template so you'd need that set up too or modify to respect whatever sensors you do have. Easiest way to implement is to use packages. That way you can just drop the whole solar.yaml into your packages folder and it will pick up all the entities.

Thanks again for the swift reply.
So I set up a packages folder as per Packages in Home Assistant (https://www.home-assistant.io/docs/configuration/packages/). Downloaded your solar.yaml and included packages in my configuration.yaml.
I need to modify my ESPHome DS18B20 temperature sensors to replace your platform: mqtt sensor. However I do not see any of the history graph or status window. Am I missing something. Thanks