In this letter, the energy efficiency (EE) for a three-node full-duplex (FD) relay channel with the amplify-and-forward (AF) scheme is studied. In particular, both the residual self-interference (RSI) and the circuit power consumption for imperfect self-interference cancellation (SIC) are considered and modeled as linear functions over the relay transmission power. Under this setup, the EE maximization problem for the AF FD relay channel is formulated and efficiently solved. Moreover, the EE performance of the direct transmission (DT), half-duplex (HD), and FD modes are compared under different channel conditions.