Wireless sensor systems represent reliable platforms for monitoring and managing of a variety of applications for health care, civil, and military environments. As most of these systems are energy constrained, an accurate control of the energy consumption is required to well design the system. The main focus of this paper is the accurate energy modeling of Wireless Body Area Networks. To this end, we discuss the propagation channel model at 2.4 GHz between two body sensors placed on the human body. This model combines path losses in free space and on human body. Then, an analytical energy consumption model is derived by including channel characteristics such as the proposed path loss model, small scale fading and signal to noise ratio. The model is validated by measurements performed with patch antennas on human subjects. Our results provide interesting insights about the effect of on-body propagation on the global path losses. Moreover, with the proposed approach, the correlation between propagation channel features and energy efficiency is highlighted.