Electricity consumption of data centers increases continuously. Beside of the IT industry which tries to reduce this consumption by improving efficiency of components in data centers, there are research solutions based on an optimized energy management of data centers by acting on the IT load placement, then on cooling, start-up and shut down. In this context, this paper focus on energetic modeling of servers in data centers. In the state of art, the IT load is usually presented as a whole unit by means of the percentage CPU, while in this work, the percentage CPU is separated in two parts. The first one is the percentage CPU due to server self applications (for example a virtual machine manager), while the second part is due to services turning on the server. This classification led to a new linear model which shows that electricity consumption of data centers can be modeled as accumulated layers depending on what kind of software is running on the servers. The model is developed and then validated with experimental measurements on actual server conduct with the help of industrial partners. This modeling presents the first step of further works aim to optimize the energy consumption of data centers by knowing the IT load that is held on its servers.