Recently, there has been a growing interest in the energy efficiency of bipedal walking robots. However, there has been no study of the effect of the spine on the overall energy consumption of robots during locomotion. This article investigates the energy efficiency of a simulated biped humanoid robot that is capable of walking with spinal motion. A systematic technique is presented to compare the energy efficiency of a robot walking with different styles of spinal movement. Simulation results show that with the additional degrees of freedom (DoF) in the torso, the humanoid robot requires 26.5% less energy than its conventional rigid-torso counterpart to complete the same walking task. Interestingly, this happens when the robot is walking with swaying hips.