This simulation-based study aimed to contribute to gaining knowledge on how much the deviation between occupant-related input by the national standard and by insitu measurements might impact predictions of heating/cooling needs in offices. Analysis was performed for two open-space offices considering 14-day-long monitoring in different seasons. The difference in simulated energy demand between the standard and measurement-based input was up to a factor of four during the heating season. Natural ventilation caused by actual windows opening increased energy demand by up to 20%. The operation of lights and blinds had a minor effect compared to the heating/cooling setpoints in buildings.