The talk presented insights from projects at the Einstein Center for the Future and the TU Berlin, both located in Berlin, on open source software and energy technologies in the German research landscape. The talk discussed how buildings impact climate, how open source can reduce that impact and the current state of open source. The talk identified obstacles to the implementation of open source, including technical, cultural, and financial. Finally, the presenter gave recommendations on how to foster an open source culture in the energy sector. These included modularized development, user-focused development, standardized interfaces, publication of data, and adapted funding. The talk concluded by stating that there has been a sharp increase in open source projects in recent years and that the future of decarbonization and digitization of the energy system requires more building stones, such as infrastructure, high-quality data, and involvement of people.