Currently, energy consumption and energy efficiency are some of the concerns of cloud system administrators. In recent years standards such as ISO/IEC 30134-4 and ISO/IEC 21836 have emerged to improve energy efficiency. Both standards are focused on the evaluation of physical servers, considering the power consumed and the maximum peak of performance, under running a SPEC benchmark. Then, server consolidation through virtualization is not considered in these standards. However, the QoS is another point that is important to consider, at the same time, by data centers administrators. In this work, we proposed a standard methodology to quantify the trade-off between the energy efficiency and the QoS of consolidated servers. As a result, it has been demonstrated through real experimentation that the proposed methodology considers the consolidation of servers in any type of virtualization environment. Besides, this methodology helps system administrators to manage cloud data centers and servers more efficiently.