The energy use is becoming a key design consideration in computing infrastructures and services. In this paper we focus on service-based applications and we propose an adaptation process that can be used to reduce power consumption. This adaptation process is materialized in an adaptation plan which fits into a software architecture specifically designed for self-adaptive systems. The adaptation plan guarantees a trade-off between energy consumption and QoS offered, while maintaining suitable revenues for the service provider. The proposed approach is based on the principle of proportional energy consumption obtained by scaling down energy for unused resources, considering both the number of servers switched on and the operating frequencies of that servers.