Teyssen's first career role was as a law clerk at the State Superior Court in
Celle, Germany. He stayed in that position until he joined PreussenElektra in
Hanover in 1989, where most recently he was Head of the Legal Department. In 1994, he earned another promotion, this time as head of legal affairs. In the course of the merger of Hastra and four other energy supply companies to form Avacon, based in
Helmstedt, His tenure was marked by decisions concerning Germany's switch to renewable energy, which he cited as a huge opportunity. In January 2012, he spoke out against the
European Union Emission Trading Scheme (EU ETS) and in 2013, he questioned the profitability of gas-fired plants. In August 2010, Teyssen and around 40 other prominent figures signed the Energy Policy Appeal for an extension of the operating lives of German nuclear power plants. In 2017, Teyssen, along with other representatives of the energy industry and Germany's Economics Minister
Brigitte Zypries, signed the agreement on financing the nuclear phase-out, which finally settled the dispute over the peaceful use of nuclear energy in Germany. June 3, 2013 Teyssen was elected President of
Eurelectric, the representative body of the European electricity industry. At the end of 2014, he announced the spin-off of E.ON SE's conventional power generation business and international energy trading to the newly founded Uniper SE, which was successfully listed on the stock exchange in 2016. In March 2018, together with
Rolf Martin Schmitz, CEO of RWE AG, he announced the reorganization of the two groups. E.ON acquired the energy infrastructure and sales operations of
Innogy SE, becoming Europe's largest energy network company and an energy supplier to about 50 million customers in more than 10 European countries. RWE acquired the renewables businesses of E.ON and Innogy, becoming one of Europe's three largest renewables players. Following approval by the antitrust authorities, the reorganization was completed in March 2020. Teyssen was Norwegian Honorary Consul General until the end of 2020, responsible for the German states of
North Rhine-Westphalia,
Rhineland-Palatinate and
Saarland. In January 2021 he was appointed as non-executive member of the Board of Directors of BP plc, in October 2021 he became Senior Advisor at
KKR, and in the beginning of 2022 he was appointed as chairman of the Board of Directors of the Swiss energy group
Alpiq Holding. ==Corporate board memberships ==