The aim of this work is to model the energy consumption and the on-site production of an existing building, the Leaf House (LH), that was designed in order to be a Net Zero CO2 emission home. For a more effective use of energy some of the most advanced available technologies in the field of renewable sources were used. In particular, the technological building plant includes several subsystems as a Geothermal Heat Pump (GHP), solar PV and thermal panels, integrated in a fully automatic heat distribution system. The building is only “nearly NZEB” and it is necessary to introduce some improvements in thermal plant and in energy production technological systems for reaching the NZEB goal.