Battery-powered wireless sensor networks (WSNs) are highly constrained in energy. In order to maximize the node and network lifetime, the energy usage has to be optimized. In this paper, we analyze the sources of energy consumption in p-persistent CSMA for non-saturated conditions, showing that maximum efficiency is achieved for a narrow range of traffic load. Furthermore, the optimum traffic load is a function of the consumption characteristics of the node and it does not guarantee maximum system throughput. Our analytical framework can be applied in various CSMA variants, thus aiding in the design of Medium Access Control (MAC) protocols for WSNs.