Dynamic voltage scaling (DVS) has been adopted in many computing systems to reduce the energy consumption of the processor by slowing down the processor speed. However, for system devices without DVS capability, the longer a task executes, the more energy the task consumes in the required system devices. This paper explores energy-efficient scheduling for periodic hard real-time tasks in a system consisted of a DVS processor and multiple non- DVS system devices. We propose an algorithm for static scheduling which minimizes the system energy consumption of a given set of real-time tasks, provided that each task executes in its worst case. For systems in which some tasks might complete earlier than its estimated worst-case execution time, we develop on-line algorithms to reclaim the slack time to reduce the energy consumption. Compared to existing algorithms, our proposed algorithm can reduce the energy consumption both in the CPU and system devices.