I installed a small power plant with a maximum power of 600 W (german so-called "Balkonkraftwerk") via a Fritz!DECT 210 power socket.
To display the actual generated power I wrote a small python-program, which runs periodically on my Raspberry Pi (cronjob). The program talks to my Fritz!Box directly to get the actual values of temperature and power and saves this values to a textfile on the Raspberry Pi.
In homebridge I use the plugin "Homebridge Temperature Humidity Sensor File" (homebridge-temperature-humidity-sensor-file v1.1.0) to read the textfiles and to display its values as sensor data.
Since there is no energy-sensor I am mis-using the "humidity-sensor" calculating the actual power as a percent of maximum power 600 W - so a value of 50% reads as 300 W.
If you are interested in this solution please let me know.
Cheers, Marc