Comprehensive studies of the energy saving potential for buildings will often not be performed in a design phase - especially not for renovation projects. There may be several reasons; for that but it may often be related to the amount of time needed to create models of the buildings and plants. The paper describes a method for auto-generation of complex simulation models for existing, documented and validated simulation programs. An automatic model generator has been developed for Danish conditions and programs. The auto-generation of complex models is based on statistical information and may rather easily be transformed to other countries and simulation programmes. Auto-generation of simulation models reduces/eliminates the user influence - leading to more consistent simulation results.