The simulation of the energy demand at the city or the district level is a research area under full development. The applications are numerous for engineers (management of heating networks, photovoltaic and shares energy…) as well as city planners or urban planning decision makers. This work thus aims at contributing to the development of methods and tools for simulation applied to urban territories. These tools are different from the actual tools which are centered on the behavior of one building in an urban site. Described methodology consists in a decomposition of the city in districts, each district being represented by a set of buildings. The electrical analogy models representing each building are assembled in a global state model and then mathmatically reduced to represent the district. These state models representing the districts are assembled and reduced to represent the city. It is necessary to adopt a quite particular structure of the state models so that these operations of assembly are possible. This procedure is described in the present paper, the results obtained on large territories are presented and they prove the effectiveness of the method.