Energy efficiency of data centers is gaining importance as energy consumption and carbon footprint awareness are rising. Green Performance Indicators (GPIs) provide measurable means to assess the energy efficiency of a resource or system. Most of the metrics commonly used today measure the energy efficiency potential of a resource, system or application usage, rather than the energy efficiency of the actual usage. In this paper, we argue that the way that the resources and systems are actually used in a given data center configuration is at least as important as the efficiency potential of the raw resources or systems. Hence, for data center energy efficiency, we suggest to both select energy efficient components (as done today), as well as optimize the actual usage of the components and systems in the data center. To achieve the latter, optimization of usage centric GPI metrics should be employed and targeted as a primary green goal. In this paper we identify and present usage centric metrics, which should be monitored and optimized for improving energy efficiency, and hence, reduce the data center carbon footprint.