Contemporary application domains make more and more appealing the vision of applications built as a dynamic and opportunistic assembly of autonomous and independent resources. However, the adoption of such paradigm is challenged by: (i) the openness and scalability needs of the operating environment, which rule out approaches based on centralized architectures and, (ii) the increasing concern for sustainability issues, which makes particularly relevant, in addition to QoS constraints, the goal of reducing the application energy footprint. In this context, we contribute by proposing a decentralized architecture to build a fully functional assembly of distributed services, able to optimize its energy consumption, paying also attention to issues concerning the delivered quality of service. We suggest suitable indexes to measure from different perspectives the energy efficiency of the resulting assembly, and present the results of extensive simulation experiments to assess the effectiveness of our approach.