This paper shows the relevance of creating an evidence based energy model as a base case to assess the energy saving potential of energy conservation measures applied to buildings. Two base case models were created using the GUI Design Builder and Energy Plus, one using this method and another using a template and assumptions based methodology. Simulation results were compared to real data obtained from the gas supplier for the years 2010 to 2012. The accuracy of the calibrated model was numerically assessed and validated using a methodology provided by three different guidelines. It was shown that the template based model had a poor accuracy, with errors up to 100% compared to real data. The calibrated model showed a significantly finer matching with the real data. An application of this model was shown through the example of external wall insulation, and difference between the two models in terms of energy savings reached up to 50 [%] for some months, possibly leading to important errors in the economic analysis of the project. This article takes part of the larger scale EU HERB project, aiming to develop a systemic approach to the retrofit of residential buildings across 7 European countries.