As energy consumption is becoming critical in Cloud data centers, Cloud providers are adopting energy-efficient virtual machines management system. These systems essentially rely on ``what-if'' analysis to determine what the consequence of their actions would be -- and to choose the best one according to a number of metrics. However, modeling energy consumption of simple operations such as starting a new VM or live-migrating it is complicated by the fact that multiple factors takes part. It is therefore important to identify which factors influence energy consumption before proposing any new model. We claim in this paper that one critical parameter is the host configuration, characterized by the number of VMs it is currently executing. Based on this observation, we present an energy model that provides energy estimation associated to VM management operation, such as VMs placement, VM start up and VM migration. The average relative estimation error is lower than 10\% using the transactional web benchmark TPC-W, making it a good candidate for driving the actions of future energy-aware cloud management systems.