template sensor for total power

config.yaml

sensor:
  - platform: template
    sensors:
      total_energy:
        friendly_name: "Total Energy"
        unit_of_measurement: 'Wh'
        value_template: >
          {% set device_a_energy = states('sensor.device_a_energy') | float %}
          {% set device_b_energy = states('sensor.device_b_energy') | float %}
          {% set device_c_energy = states('sensor.device_c_energy') | float %}
          {{ (device_a_energy + device_b_energy + device_c_energy) | round(2) }}