Energy consumption and thermal performance are the two most important tasks in data centers (DCs) facility management. In recent years, to monitor and control their variation several performance metrics were introduced. In this paper an overview on the main important energy and thermal metrics is provided. A critical analysis to investigate mutual relations among metrics was performed, with the aim to clarify some physical aspects regarding the assessment of DC global energy performance. Indeed, although these metrics are commonly used to assess the energy efficiency of DCs, their usefulness for encouraging lower energy consumption was poorly investigated. Moreover, an analysis on the effect of the DC thermal performance on metrics was done. The thermal management assume a key role for achieving energy saving during the operation of a DC and for the improvement of the IT equipment reliability.