I've had my PV system and storage system from TK-Elektrotechnik for two weeks now and I'm extremely satisfied.
Both the technology used and the knowledge of the bosses and employees are, in my opinion, state of the art. The installation was carried out cleanly, quickly and professionally.
In my opinion, it was very good that all the work was carried out in-house (i.e. installation of the panels, cables, storage system, inverter, etc.) and not subcontracted out.
I always had the feeling that I was being looked after by a team that really enjoyed their job and did it very accurately. They were very flexible in responding to my wishes, even when I expressed them relatively spontaneously while the installation phase was already underway, and it always seemed to me that the company was really interested in building me a sensible, high-quality and practical solution.
Therefore, I would like to take this opportunity to thank them once again for their great commitment! I hope I can make many more recommendations for TK-Elektrotechnik; in my view, a real highlight in the field of electrical engineering and renewable energies.
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Translation
Comment from Janssen Sanitär-und Heizung GmbH: 
Thank you for the good evaluation.
We also enjoy working with you.
MFG
The Janssen Team