Energy is a major concern in wireless networks, be it for the target mobile user, as is the case of IEEE802.11 Infrastructure mode, or for all the users as in IEEE802.11 ad hoc mode where it dictates the lifetime of the whole network. We focus in this paper on the modeling of energy consumption at the MAC layer in the context of QoS-capable WLAN, namely IEEE802.11e, and propose an analytical model for the lifetime of a terminal for different access categories. Our results quantify the impact of priority on energy consumption and show how the latter varies as a function of the network size, the number of stations in each priority group as well as the propagation environment : indoor versus outdoor. These results can be used at higher layers to optimize energy consumption through differentiation.