In this paper, the authors present a global study on energy considerations in the context of a wireless network based on the well-known IEEE 802.15.4 technology. Various tests have been carried out on a node prototype powered by Lithium batteries in order to determine the lifetime of this network element, in several cases including 802.15.4 coordinator or end device configurations but also non-802.15.4 protocols. The results show that the lifetime is clearly dependent on the node receiving state time, which has a big impact on not only the network performance like throughput and latency but also the energy budget of the battery. The paper also illustrates the non-linear characteristic of the battery capacity. Finally, it identifies drawbacks which may be avoided in the future design of MAC (Medium Access Control) protocols in order to maximize node lifetime.