The article explains the stages of a research job aimed at establishing an analytical methodology to analyse the energy performance of existing buildings through the construction of Energy Signature, a monitoring method in which measured energy consumptions are related to local climatic variables, such as external temperature. The energy consumption of buildings represents a large percentage of total consumptions of a nation; these are often also higher than those actually needed to ensure optimal conditions for the comfort of the zone. The causes are related both to typological defects, such as housing poor insulation, thermal bridges, poorly designed heating system, and to a wrong behaviour of final users. Energy Signature has a twofold purpose: on the one hand it can be used as a pre-energy audit by a professional to highlight the glaring problems, on the other hand it can be a valuable tool for raising end users' awareness. The goal of the present treatment is to define a new accurate calculation method that implements the EN 15603, aimed at characterizing, from a technical point of view, the Energy Signature in objective and univocal way. This proposal is defined through an holistic and multidisciplinary approach that apply statistic, math and geometry methods for Energy Signature construction, called MESH.