The paper presents a simple probabilistic analysis of the energy consumption in preamble sampling MAC protocols. We validate the analytic results with simulations. We compare the classical MAC protocols (B-MAC and X-MAC) with LA-MAC, a method proposed in a companion paper. Our analysis highlights the energy savings achievable with LA-MAC with respect to B-MAC and X-MAC. It also shows that LA-MAC provides the best performance in the considered case of high density networks under traffic congestion.