In this paper, an approach that uses Dynamic Voltage Scaling (DVS) to reduce the configuration energy of runtime reconfigurable devices is proposed. The basic idea is to use configuration prefetching and parallelism to create excessive system idle time and apply DVS on the configuration process when such idle time can be utilized. A genetic algorithm is developed to solve the task scheduling and voltage assignment problem. With real applications, the results show that up to 19.3% of configuration energy can be reduced. When considering the reduction of the configuration energy, the results show that using more computation resources is more favorable when the configuration latency is relatively small, and using more configuration controllers is more favorable for relatively large latency.