The Energy Performance Certification (EPC) of existing residential buildings has been designed to introduce energy efficiency as a comparative criterion for real-estate purchase choices, which should influence real-estate market value and stimulate energy saving investments. EPCs in Belgium are asset ratings, calculated with a standardized approach, which purposefully (and understandably) takes the human factor out of the equations in order to allow the comparison of buildings. As a result, they often overestimate energy consumption and present discrepancies that do not allow appropriation of the results by potential buyers. This study proposes complementary EPC results, obtained by integrating the behavior of occupants in the calculation method. This paper first analyses the pool of behavioral uncertainties that influence the results of EPCs, describes a questionnaire built to gather additional data on households’ characteristics and energy consumption habits and proposes modifications to the calculation method. The complementary results are then compared to regulatory EPC results and real consumption data. Though acknowledging the necessity of a standardized EPC for dwellings’ comparison, this study completes it with data on the energy consumption-related behaviors to (partially) close the gap between real and theoretical consumptions.