European directive EPBD (2003) and Kyoto protocol (1997) obligated the signer countries to implement an energy certification process of buildings in their laws. In Spain, the government established (RD 47/2007) two certification options: a simplified certification (that does not permit to obtain the best certification results) or a simulation process based on the Lider and Calener tools (DOE-II based simulators adapted to Spanish norms). In this work we report the results of a large number of residential building simulation and certification processes, obtained during years 2007, 2008 and 2009. Architectural factors that may improve or affect the certification result are identified and discussed by comparative analysis. Residential buildings investigated are grouped in tree typologies: familiar house, little dwellings block (minus than 10 dwellings) and medium dwellings block (between 10 and 30 dwellings). For each group the more relevant architectural factors are discussed. All the buildings locate in the Autonomic Community of Catalonia, in a Mediterranean climate. At the end of the work a discussion between the different groups is conducted. Finally, certification process is critical discussed, especially thinking in the repeatability of the results and in the capacity of the analysed tools to express an architectonic result, in terms of geometrical distribution, form, magnitude of the buildings. Alternatives to the actual Lider and Calener simulation process are also critically discussed.