This work is motivated by the needs of energy-efficient designs for multi-context FPGAs. Dynamic-voltage-scaling-based algorithms are proposed to schedule loading and executing of tasks in a multi-context FPGA. When a task partition over contexts is given, two optimal scheduling algorithms are proposed in minimizing the energy consumption and the maximum time span of task executions. When no task partition is given, two approximation algorithms with approximation ratios 2.371 and 1.540 are presented in the energy-consumption minimization and the maximum-time-span minimization, respectively.