In this paper, we introduce a discrete-time model for the complete power supply sub-system that closely approximates the behavior of its circuit-level (i.e., HSpice), continuous-time counterpart. The model is abstract and efficient enough to enable event-driven simulation of digital systems described at a very high level of abstraction and that include, among their components, also the power supply. Therefore, it can be successfully used for the purpose of battery lifetime estimation during design optimization, as shown by the results we have collected on a meaningful case study. Experiments prove also that the corresponding Spice-level model very close to that provides the accuracy of our model.