Consistent hash based storage systems are used in many real world applications for which energy is one of the main cost factors. However, these systems are typically designed and deployed without any mechanisms to save energy at times of low demand. We present an energy conserving implementation of a consistent hashing based key-value store, called PowerCass, based on Apache's Cassandra. In PowerCass, nodes are divided into three groups: active, dormant, and sleepy. Nodes in the active group store cover all the data and running continuously. Dormant nodes are only powered during peak activity time and for replica synchronization. Sleepy nodes are offline almost all the time except for replica synchronization and exceptional peak loads. With this simple and elegant approach we are able to reduce the energy consumption by up to 66% compared to the unmodified key-value store Cassandra.