Switzerland has adopted the concept of the 2000 Watts society as a long term target, with an intermediate objective for 2050: a reduction of the average energy consumption per person from 6300 to 3500 watts, including a maximum of 2000 watts of non-renewable energy. Following this concept, the SIA (Swiss Society of Engineers and Architects) has developed energy consumption targets for the built environment: embedded energy of materials, operating energy (heating, domestic hot water, electricity, air conditioning) and mobility. In order to achieve these targets in all projects of new buildings or renovations, the Canton of Vaud and the City of Lausanne have adopted and included them into SméO, a decision-making tool for the sustainability assessment of building or neighborhood projects. This paper presents the methodology applied by the tool for the energy assessment. The methodology is entirely based on building standards and norms for all the domains where these exist. In the other cases, assumptions have been made. The evaluation follows the entire project. For the first stages of the project where energy calculations have not been done yet, the tool offers default values for energy consumption, mobility and embedded energy based on the characteristics of the project. Hence, the project can be optimized from the very beginning of the process. SméO being a free web platform, results can be shown and easily communicated to all stakeholders.