With the development of MEMS and CMOS technologies, the implementation of a huge number of wireless distributed micro-sensors that can be simply and rapidly deployed to form highly redundant, self-configuring, and ad hoc sensor networks. Sensor nodes are generally battery- devices, the critical facets to face concern are how to minimize energy consumption of nodes, so that the lifetime of sensing nodes can be maximized. The first step to achieve this goal is to know completely the sources of energy consumption in wireless sensor networks. In this paper, sources of energy consumption at various communication layers have been studied and investigated. Moreover, the energy consumption for the components of a typical sensor node and the impact of communication protocols stack on the energy consumption are discussed. In the sequel, the sources of energy consumption in each communication layer individually are studied. Then, a survey has been provided for existing energy models and the classification of these models into physical layer, MAC layer and cross-layer energy models