The development cost of low-power embedded systems can be significantly reduced by reusing legacy designs and applying proper modifications to meet the new power constraints. The proposed power management techniques in the literature for implementing distributed power managers in multi-processor systems are very costly in terms of hardware and software modifications. For example, extra software code for power management must be integrated into each component of the system. Furthermore, in order to turn on a component at a specific time/event, extra hardware timers and interrupt controllers must be added to each component along with proper software/device driver modifications. In this paper, we propose a new centralized power management technique that reduces the power consumption of distributed realtime systems at very low implementation cost. Our power manager does not need software and hardware modifications of each individual component. Instead, it uses the model of the system/application to compute the schedule of turning on/off commands by dynamically simulating the system for a given application scenario. The dynamic simulation can be conservative to reflect the jitter in arrival time of events and/or variation in execution delay of tasks. We applied our power management technique to a distributed software-defined radio system and achieved significant energy savings (60% to 87%) at the cost of 1% energy consumed by the power manager itself, as verified by actual hardware measurements. Furthermore, our power manager reacts to the changes in application scenario (referred to as mission) within milliseconds.