Ha. Oh dear. This only updates when the doorbird event occurs.
template:
- trigger:
- platform: event
event_type: "doorbird_turstation_klingelt"
binary_sensor:
- name: doorbell_is_ringing
state: on
auto_off: "00:00:30"
device_class: occupancy
- name: lichtschalter_eingang
state: "off"
device_class: light
sensor:
- name: "Justiert"
unit_of_measurement: "W"
state_class: measurement
device_class: power
state: "{{ states('sensor.sensor.tasmota_energy_power')|float(0) - 8 }}"
availability: "{{ states('sensor.sensor.tasmota_energy_power')|is_number }}
Try pressing your doorbell - the energy sensor will update 
What you want is this:
template:
- trigger:
- platform: event
event_type: "doorbird_turstation_klingelt"
binary_sensor:
- name: doorbell_is_ringing
state: on
auto_off: "00:00:30"
device_class: occupancy
- name: lichtschalter_eingang
state: "off"
device_class: light
- sensor:
- name: "Justiert"
unit_of_measurement: "W"
state_class: measurement
device_class: power
state: "{{ states('sensor.sensor.tasmota_energy_power')|float(0) - 8 }}"
availability: "{{ states('sensor.sensor.tasmota_energy_power')|is_number }}