Star-Cap is a high-fidelity, real-time power management system for computer clusters. Star-Cap’s cluster power management module uses software-only power models to implement a proactive power capping mechanism with the ability to distribute a cluster’s power budget non-uniformly across nodes. We evaluated Star-Cap on a variety of MapReduce-style workloads running on low-power mobile and server systems. Depending on application and plat-form, Star-Cap improves cluster throughput by 14-43% compared to using a uniform power cap distribution policy with a purely reactive power capping mechanism. Furthermore, Star-Cap maintains cluster throughput while reducing overall cluster power budget by 14% with no additional capital or operating cost. Star-Cap’s proactive power capping mechanism also improves system response time to power cap violations by a factor of 2 to 10. Star-Cap’s overhead for collecting, computing, and reporting data is less than 1% CPU utilization.