Background Citizen energy cooperatives (CECOs) are an important element for realising the transformation of the European energy system from a central system, dominated by a few market players to a decentralized system with the participation of many citizens, in which energy supply is largely based on renewable energy sources. This article identifies success factors that ensure a working organization that is capable of implementing projects to support the energy transition and improving the well-being of citizens. Methods The aim of this study is to identify success factors using the example of CECOs in north western Germany through a two-step process. The first step is a literature review to identify success factors of CECOs. Second, 12 semi-structured expert interviews with managers of CECOs are conducted and evaluated by means of a qualitative content analysis. The interviews identified the success factors and barriers of CECOs. The interviews were recorded and transcribed verbatim. The coding process was carried out using a qualitative data analysis software (ATLAS.ti 8.4.5,). Results The results show that three factors particular should be present, but can influence each other, to lead to successful CECOs: (1) the CECOs require business models with low complexity, as business models that span different stages of the value chain can overwhelm organizational managers. (2) Committed key individuals must be found and promoted. They need the support of the members. (3) Social and ecological principles are the core values of CECOs. The maintenance and strengthening of social–ecological credibility must be promoted in the public perception. Conclusions The results show that CECOs emerge from a strong social cohesion in a society. In addition, local "key individuals" need to be identified and supported. To this end, the framework conditions (e.g., policy, legislation, municipal support, etc.) must be adapted in such a way that these "key individuals" in CECOs are not overburdened. The analysis of the interviews also revealed that political or regulatory barriers are often cited as obstacles to the implementation of community energy projects.