In this paper, we address the energy ef ciency analysis of the relay channel under ergodic fading. The study considers full duplex and half duplex terminals. Since the capacity of general relay channels is unknown, we investigate achievable rates with decode and forward and capacity upper bounds with the cut-set bound. The maximum rate per energy and the slope of the spectral ef ciency with the energy per bit are computed to asses the impact of the duplexing capabilities, the resource allocation and the channel fading distribution.