This paper presents the design and implementation of a multi-standard energy management system, which leverages heterogeneous devices to convert existing buildings into Smart Buildings. Its main purpose is to increase the energy efficiency of buildings providing user awareness to promote green behaviors. The proposed solution has been designed to enable interoperability across different standards and protocols in order to develop applications with which end users can interact with the system. Finally, a web portal and a smartphone application to give feedback and to view environmental information are presented.