The IEEE 802.15.4 standard is considered as the most notorious MAC layer for wireless sensor networks (WSNs) in both centralized and distributed context. For instance, in multi hop environment, the beaconless IEEE 802.15.4 is used. Several works evaluated the performance of the beaconless IEEE 802.15.4 in terms of average delay, average energy consumption, throughput etc. But, none of the existing studies derived accurate energy consumption bounds of this MAC layer. In this paper, our contribution is twofold. We first propose a comprehensive energy consumption analysis of the unslotted CSMA/CA algorithm. The results are validated through simulation. Then, we exploit our analysis to propose a cross layer routing scheme that enhances the native PEGASIS protocol. Our scheme called Average Energy Enhanced PEGASIS (AE2-PEGASIS) considers the average energy consumption at the MAC layer when constructing the routes to the sink.