Power management saves power by shutting down idle devices. These devices often serve requests from concurrently running tasks. Ordering task execution can adjust the lengths of idle periods and exploit better opportunities for power management. This paper presents an on-line low-power scheduling algorithm for multiple devices. Simulations show that it can save up to 33% power and reduce 40% state-transition delays. This algorithm is robust under imperfect knowledge of future requests and timing constraints; therefore, it is applicable to interactive systems.