One of the great societal challenges that we face today concerns the move to more sustainable patterns of energy consumption, reflecting the need to balance both individual consumer choice and societal demands. In order for this "energy turnaround" to take place, however, reducing residential energy consumption must go beyond using energy-efficient devices: More sustainable behaviour and lifestyles are essential parts of future "energy aware" living. Addressing this issue from an HCI perspective, this paper presents the results of a 3-year research project dealing with the co-design and appropriation of a Home Energy Management System (HEMS) that has been rolled out in a living lab setting with seven households for a period of 18 months. This HEMS is inspired by feedback systems in Sustainable Interaction Design and allows the monitoring of energy consumption in real-time. In contrast to existing research mainly focusing on how technology can persuade people to consume less energy ("what technology does to people") our study focuses on the appropriation of energy feedback systems ("what people do with technology") and how newly developed practices can become a resource for future technology design. Therefore, the authors deliberately followed an open research design. In keeping with this approach, this study uncovers various responses, practices and obstacles of HEMS use. They show that HEMS use is characterized by a number of different features. Recognizing the distinctive patterns of technology use in the different households and the evolutionary character of that use within the households, they conclude with a discussion of these patterns in relation to existing research and their meaning for the design of future HEMSs.