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In this paper, we study the problem of reducing the energy consumption for hard real-time systems based on fixed-priority (FP) scheme. To balance the static and dynamic energy consumption, the concept of critical speed was proposed. Moreover, when combined with the processor shutdown strategy, the critical speed was widely used as the lower bound for voltage scaling in literature. In this paper, we show that the critical speed strategy might not always be more energy efficient than the traditional DVS strategy and there is a tradeoff between these two strategies. Based on it, we propose an off-line approach to set up the energy efficient static speed schedule for real-time tasks. The simulation results demonstrate that our proposed techniques can effectively reduce the energy consumption for hard realtime systems.