A system for controlling power usage in a storage cluster by dynamically controlling membership in the storage cluster is disclosed. The storage cluster includes multiple storage servers that provide access to one or more storage subsystems. The power management system uses a power management policy to set parameters for controlling membership in the storage cluster and monitors the storage cluster based on the policy. Based on the monitoring, the system detects when the number of storage servers in the storage cluster should be reduced or increased. To reduce the number, the system selects a storage server to deactivate and directs the selected storage server to migrate storage resources (e.g. data, metadata) associated with the server to a different storage server. The system then deactivates the selected storage server by directing it to transition to a low power mode. The system may increase the number of servers in the storage cluster by reversing these steps.