Within sustainable HCI research, we see a growing interest to study how designing interactive technology can improve the utilisation of renewable energy resources. In this case study, we explore the concept of energy communities and how technology can be designed to support people to cooperate around transitioning to a more sustainable use of electricity. To do so, we designed the Lumen prototype, which aims to support a small energy community in shifting domestic energy-consuming practices to align with times of high availability of sustainable energy. By creating awareness of current and future sustainable energy availability through an ambient feedback display, the Lumen prototype informs households about the community's consumption patterns. To obtain insights into how people understand and experience an energy community, we conducted a qualitative field study with three Danish households. Through our study, we found sustainable awareness and incentives materialised in the ambient display were amplified by the dynamics of the community. We conclude by discussing future directions for exploring how to design technology for energy communities.