Please format your config for the forum correctly, https://community.home-assistant.io/t/how-to-help-us-help-you-or-how-to-ask-a-good-question/114371
You will need an availability template if you ever want to use the sum sensor in calculations or the energy dashboard.
You got the energy sensor device class wrong.
You can use capital letters and spaces in the names. They will look better on your dashboards. These will have exactly the same entity ids:
sensor:
name: "Energia Sum Inverters kWh"
unique_id: "sensor.energia_sum_inverters_kWh"
unit_of_measurement: "kWh"
device_class: energy
state_class: total_increasing
state: "{{ states('sensor.inverter_inverter_inverter_15kva_energy') | float + states('sensor.inverter_inverter_inverter_5kva_energy') | float }}"
availability: "{{ has_value('sensor.inverter_inverter_inverter_15kva_energy') and has_value('sensor.inverter_inverter_inverter_5kva_energy') }}"
sensor:
name: "Inverter Sum Energy Inverter W"
unique_id: "sensor.energy_sum_inverters_W"
unit_of_measurement: "W"
device_class: power
state_class: measurement
state: "{{ states('sensor.inverter_inverter_15kva') | float + states('sensor.inverter_inverter_5kva') | float }}"
availability: "{{ has_value('sensor.inverter_inverter_15kva') and has_value('sensor.inverter_inverter_5kva') }}"