Adding template sensors to the GUI has been asked for previously so I am closing this as a duplicate.
Feel free to vote here: Add and edit template entities through helper entities page
To solve your request in the meantime you can create a template sensor like this:
template:
- sensor:
- name: "Gira Switch Power"
unit_of_measurement: "W"
device_class: power
state_class: measurement
state: "{{ states('sensor.your_mah_sensor_here')|float(0) * states('sensor.your_voltage_sensor_here')|float(0) * 1000 }}"
availability: "{{ states('sensor.your_mah_sensor_here')|is_number and states('sensor.your_voltage_sensor_here')|is_number }}"
Also, the Energy dashboard does not support Power sensors.