SONNET aims to create an inter- and transdisciplinary understanding of the diversity and processes of social innovations in the energy sector (SIE). It assesses - critically and reflexively- the success, contributions and future potential of SIE towards sustainable energy transitions in Europe. SONNET investigates how, to what extent and under which enabling conditions diverse types of SIE may result in new breakthroughs or successfully help to overcome transition barriers; such as limited citizen engagement or slow adoption of new technologies. SONNET’s empirical research is informed by a novel conceptual framework combining insights from sustainability transitions, energy studies and social innovation literatures. It bridges qualitative and quantitative methodological approaches in an innovative multi-method research design. Across 30 qualitative in-depth case studies situated in six European countries, SONNET investigates the diversity, processes, success and contributions of SIE. Given its focus on urban areas as major hubs for SIE, SONNET conducts six transdisciplinary SIE city labs to experiment with new forms of SIE and learn about how multiple actors can harness the potential of SIE. In addition, based on three large-scale representative citizen surveys, SONNET assesses the future potential of SIE and derives implications for reconfiguring existing and developing new business models. SONNET synthesizes its findings in an integrated knowledge framework for a socio-economic, socio-cultural (incl. gender) and socio-political understanding of enabling and impeding conditions for SIE and SIE contributions when working towards accelerating sustainable energy transitions in Europe. Through a cutting edge co-creation, dissemination and exploitation strategy, SONNET ensures that its practical recommendations, tools and capacity building activities have a maximum impact on its key stakeholders such as citizens, SIE actors, policy makers, and businesses.