Modeling simplification related to occupant’s behavior is a major cause of gap between actual and model’s predicted energy use of buildings. This paper aims to identify those parameters of realistic occupants-related heat gains that actually cause this gap. The investigation therefore, systematically distinguishes the occupant behavior using three behavior parameters, namely: the occupancy behavior, the appliance use behavior and the family size. The effect of these parameters is investigated on a building for two different insulation standards using heat pump as energy supply system. The results identifies the occupancy patterns and the household size as two major parameters that explains a large portion of the gap between actual and model’s predicted energy use of the building. Results further show that variation in household sizes is an important parameter to understand the variation in the actual energy use for similar buildings. The study also shows a clear influence of occupant’s behavior on the performance of heat pumps and pinpoints the variations in share of space heating needs compared to domestic hot water needs as a major cause for this influence. Sensitivity of findings is tested against building thermal mass and condensing gas boiler. Analysis shows no significant variations in the conclusions. The study therefore concludes that using identified parameters in modeling practices can contribute to improve the prediction of actual energy use of buildings.