This article presents the results of a three-year (Nov. 2010–Dec. 2013) model test that was part of the “Occupational Training for a Sustainable Development” program sponsored by the German Bundesinstitut für Berufsbildung (Federal Institute for Vocational Education and Training). It was conducted by the University of Oldenburg’s department of vocational training and business education jointly with the Bundestechnologiezentrum für Elektro- und Informationstechnik e. V. (BFE) (Federal Center for Electrical Engineering and Information Technology). The model test pursued two different, albeit connected research approaches: a concept-development and a theory-modelling approach. The first one successfully developed, implemented and evaluated the unique, flexible and modularized renewable energies/energy efficiency specialist advanced training as defined by §42a of the German Chamber of Crafts’ trade and crafts code. The second stream of theory-modelling complemented this by gaining well-founded insight that helps to describe and explain how exactly an occupational training works for sustainable development in professions operating in the energy sector.