In this paper, we are conducting a probabilistic analysis of the energy consumption of nodes of a wireless sensor network caused by the use of MAC layer protocols, to know which one is best suited to save energy. Representative examples of duty-cycled protocols have been chosen: S-MAC as a pure contention-based protocol, and Eco-MAC as a hybrid protocol, i.e. a protocol which uses contention and multiplexing mechanisms. We have chosen simple network topologies to highlight the role of collisions in energy expenditure, and the way the previous protocols treat them. We have carried out our analysis by using the stochastic process algebra PEPA. Its formal language has been used to model networks which implement these protocols. The PEPA Eclipse plug-in permitted us to simulate the models and to show that Eco-MAC saves energy of nodes in a better way than S-MAC, for the network topologies we modeled. For further information, please visit this web site.