This paper presents the specification of a context-aware system dedicated to assist home occupants in their everyday life while reducing their energy consumption. The system behavior and the interaction are built upon the definition of "situation spaces" based on a prior definition of the contexts of activity from the point of view of each actor in the home, i.e. taking into account actors' concerns. The interaction specification appears to be a way to manage the discrepancy between users' concerns and the system context, which can reduce errors. To develop context-aware systems that can easily be appropriated and thus potentially "invisible," we believe it is essential to articulate choices about architecture and interaction with models of individual-collective activities built upon real-life observations.