Purpose: This study aims to investigate and compare a range of factors that can contribute to differences in energy calculations and the energy declaration. Method: To investigate this, a literature study has been carried out where the collection of facts and information is used to complement the case study that has also been carried out. The case study consists of energy calculations that have been both calculated and compared for a larger and a smaller one-family house. To examine the impact of the various factors, one factor at a time has been changed in the calculations to clarify its outcome. To make the work as realistic as possible, two existing houses are used as reference objects. This means that both floor plans and material choices are taken from a house manufacturer’s drawings. Results: The result shows that living habits are a difficult factor to assess when it comes to energy use in a building because there are no specified values for each sub-item. The energy use that can be read out is a total sum of the different parts of the energy use. Examples of parts that are included in the total energy use are domestic hot water use and household electricity use. Despite the difficulty in distinguishing which item is the major contribution factor to the differences in measured and projected energy use, most agree that living habits are the biggest factor. Conclusions: It becomes easier to identify what it is in living habits that is the difficult-to-assess factor if all individual living habits were specified with their own energy calculations and measurement values. This could even be a tool to investigate and further investigate the differences that arise in the future. Finally, it can be good to assist with skill development for all homeowners in terms of energy efficiency in their living habits to reduce ignorance about the impact of user habits on energy use.