Recently the concern on energy efficiency in wireless communications has been growing rapidly as energy consumption increasingly becomes a global environment problem. Lots of research and development efforts have been spent in wireless industry, aiming for energy efficient solutions which lead to green wireless communications. In this paper we provide a brief overview on those efforts, with an emphasis on introducing energy efficiency metrics. Since energy efficiency metrics are indicators of efficiency, understanding those metrics provides us a better view on how energy efficiency can be achieved in wireless systems/networks. Observing energy is a concern at every corner of a wireless system/network, we describe those metrics from at component, equipment and system/networks level, respectively. From our observation, energy efficiency metrics for components and equipments have been well established. However, studied for system/network level solutions and metrics deserve more attention. We believe new energy efficient architecture and associated metrics will be the key for the energy consumption problem in wireless industry.