This paper presents analysis methods for energy estimation in RC trees driven by time-varying voltage sources, e.g., buffers, timevarying power supplies, and resonant clock generators. An Elmore energy model that is the computational analog of the conventional Elmore delay model for RC trees is described. Simulation results indicate that the error in energy estimation is less than 2.5% in the worst-case in comparison to HSPICE simulations, with over a 1000X speed-up. Categories and Subject Descriptors: B.7.2 [Integrated circuits]: Design aids--simulation General Terms: Algorithms