The promotion of the exploitation of renewable sources in the built environment has led to the spread of multi-energy systems in buildings. These systems use more than one energy source in various energy converters to overcome the limitations that may be characteristic of each source. However, the design of the optimization of such systems is a complex task because the number of design variables is high and the boundary conditions (climate, operation strategies, etc.) are highly variable, so the system simulation has to be performed in the time domain. In this work an original hourly model to optimize multi-energy systems is presented and applied on a case study. It is an evaluation method to assess, in an integrated fashion, the performance of a building system as a whole and the viability of the exploitation of various energy sources. This tool is intended to take into account the variation of the conversion efficiency as a function of the design power. part load, boundary and climatic conditions. The relations that can model the energy converters of the case study (standard boiler, condensing boiler, various types of chillers and others) from the energy performance and from the financial points of view are also presented. This model represents a valuable alternative to currently available tools for hybrid systems simulation because of the optimization approach and of the detail in the thermal energy converters performance. Ultimately, the theoretical and applied knowledge of this contribution aims also at promoting a more conscious use of renewable and non-renewable energy in the built environment. (C) 2009 Elsevier B.V. All rights reserved.