In the last seven years, a method has been developed to analyse building energy performance using simulation, in Brazil. The method combines analysis of building documentation, and walk-thought visits, electric and thermal measurements, with climate analysis and the use of an energy tool (DOE2.1E code). The method was used to model more than fifteen office buildings (more than 200 000 m²), between 12.5 and 27.5° of South latitude. The paper describes the basic methodology and justifies it, using some results.