Academic research and policy have focused on sustainable energy transitions for addressing the societal challenge of climate change for a long time, but the concept of ‘social innovation’ has only been recently taken up. This concept refers to different social phenomena relating to changes in socio-technical systems. Moving beyond narrow perspectives on social innovation, this article asks how we can capture the diversity of social innovation, taking the example of the energy sector. It proposes a comprehensive typology of social innovation that allows to capture the phenomenon in its empirical diversity, and to more systematically investigate processes of social innovation and their contributions to making socio-technical systems more sustainable. The typology is based on a conceptual understanding of social innovation in energy (SIE) as comprising changing social relations involving new ways of doing, thinking and/or organising energy. It is empirically grounded in mapping and analysing 500 SIE-initiatives across eight European countries. Such a conceptually-informed and empirically-grounded understanding of SIE sheds light on the ‘social’ as an object of innovation, the agency of a multitude of actors, and the different types of social relations and activities pursued by energy system actors. It also opens the possibility to publicly discuss the diverse social innovations and their interdependencies, (un)desirability, as well as transformative potentials in energy transitions.