This was my new style template for my whole house energy meter :
template:
- sensor:
- name: "House electricity energy"
unit_of_measurement: 'kWh'
state: >
{{ (states("sensor.mains_consumed_energy") | float + states("sensor.upstairs_consumed_energy") | float) + (states("sensor.extra_consumed_energy") | float) | round(2) }}
device_class: energy
state_class: measurement
I tried to add a last_reset, but that results in an error when checking config. That is why I had to add it under homeassistant in configuration.yaml.
It seems the developers forgot to add that important piece to the new template style when creating the new energy function.