Crab walking is the most general and very important one for omni-directional walking of a hexapod robot. This paper presents a dynamic model for determining energy consumption and energy efficiency of a hexapod robot during its locomotion over flat terrain with a constant crab angle. The model has been derived for statically stable crab-wave gaits by considering a minimization of dissipating energy for optimal foot force distribution. Two approaches, such as minimization of norm of feet forces and minimization of norm of joint torques have been developed. The variations of average power consumption and energy consumption per weight per traveled length with velocity or stroke have been compared for crab walking with tripod and tetrapod gait patterns. Tetrapod gaits are found to be more energy-efficient compared to the tripod gaits.