Modelling the energy use of buildings during the planning process is a well-established practice within the construction industry today. This article studies how these models are handled in practice and the issues that arise around them. This is a case study that follows the planning process of a block of rental buildings in Sweden. With an Actor Network approach this article shows how the complexity of the energy model affects the relationships between the energy consultant and the professionals from the construction company. Since the construction company professionals do not understand the calculations behind the model, they have to trust the energy consultants expertise. Furthermore, the energy modelling practices create tensions when proposed architectural designs are at odds with the energy efficiency goals. Lastly, the article shows how the uncertainties connected to the models calculations provide an arena where personal feelings are allowed to be part of the process. From the perspective of the involved professionals, energy modelling is shown to entangle social and material relations in ways that have not previously been studied in relation to energy efficiency in the process of planning new buildings.