The first step of my plan, building a Raspberry Pi based photovoltaic monitoring solution, is finished. I created a python package that works with the Kostal Piko 5.5 inverter (and theoretically should work with other Kostal inverters as well) and offers a clean interface for accessing the data:
#create a new piko instance
p = Piko('host', 'username', 'password')
#get current power
#get voltage from string 1
The next steps are collecting the data and processing it.