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Energy consumption is an important performance parameter for portable and wireless embedded systems. However, energy consumption must be carefully balanced with real-time responsiveness in hard real-time systems. In this paper, we present two offline dynamic voltage scaling (DVS) schemes for dynamic power management in such systems. In the first method, we develop a generalized network flow (GNF) model for the uniprocessor DVS problem and solve it optimally using an efficient network flow algorithm. The proposed method outperforms existing DVS schemes for several popular embedded processors where the number of processor speeds is limited to a few values. The solutions for the GNF model provide theoretical lower bounds on energy consumption using DVS in hard real-time systems. We also describe a minimum-cost network flow model whose solutions are near-optimal. The minimum-cost models perform at par with competing methods for processor models with a large range of operating voltages, and better than them for processor models with a limited set of operating voltages.