An energy balance over an urban area and over a rural one, reveals that the first case is always more complex than the last one. The urban context is able to change all the energy transferences mainly due to its own layout. Thus, analysing all the energy transferences one by one, we can find how they affect in a different way depending on the surroundings, among other influences. The combination of all the differences between an urban and a rural area, from an energy point of view, are responsible of the urban overheating mainly observed in the evening, what is well known "uban heat island" effect. It is, therefore, necessary to find the interrelations among all the mechanisms of heat transfer in an urban area, and in this way to be able to assess the influence of the urban context on building energy demand.