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:

import pikopy
#create a new piko instance  
p = Piko('host', 'username', 'password')

#get current power  
print p.get_current_power()

#get voltage from string 1  
print p.get_string1_voltage()

The next steps are collecting the data and processing it.

The library can be downloaded from https://pypi.python.org/pypi/pikopy

The code is on Github: https://github.com/Tafkas/KostalPikoPy