A significant amount of energy in the buildings can be saved by inducing efficient occupant behavior. The occupant's awareness tools that have been shown to be effective in achieving energy efficiency gains depend on various computational and estimation algorithms. This paper proposes an energy feedback scheme that relies on a model based, building thermal simulation in order to identify the areas for efficiency improvement. By leveraging the specific mathematical formulation of those models and a dedicated open-source solver, improved computational speed, reduced cost and enhanced interoperability is obtained. This combined with the integration into a building management system (BMS), permits real-time sensing and feedback. Unlike similar studies, this work's outcome allows the creation of the energy awareness tools that rely solely on validated thermal model simulation, thus increasing their accuracy and potential in the future smart buildings.