With growing cost of electricity, the power management of server clusters has become an important problem. However, most previous researchers only address the challenge in homogeneous environments. Considering the increasing popularity of heterogeneous systems, this paper proposes an efficient algorithm for power management of heterogeneous soft real-time clusters. It is built on simple but effective mathematical models. When deployed to a new platform, the software incurs low configuration cost because no extensive performance measurements and profiling are required. To strive for efficiency, a threshold-based approach is adopted. In this paper, we systematically study this approach and its design decisions.