I got It to work!
For the fake power meter I use:
- platform: template
sensors:
bedroom_heater_power:
friendly_name: "Bedroom Heater Power"
unit_of_measurement: 'kW'
value_template: >
{% if is_state('switch.your_power_plug', 'on') %}
1.50 # <- replace with the kW of your device
{% else %}
0
{% endif %}
And this for the integration sensor:
- platform: integration
source: sensor.bedroom_heater_power
name: Bedroom Heater Power Consumption
round: 2
Thanks for the solution @nickrout !