Even if the massive wireless backhaul traffic can be transmitted back to the core network with a specified QoS, it is a key challenge to deploy it in a highly energy-efficient way. Some studies indicate that densely deployed low-power BS will decrease the energy consumption in 5G networks. However, based on our analytical results, different architectures of backhaul networks have different energy efficiency models. For example, in the central solution, the energy efficiency of wireless backhaul networks reach a saturation limit when the density of small cells is larger than a specified threshold. Optimizing the energy efficiency of wireless backhaul networks is very important for future 5G networks. Some potential solutions are presented to face this energy efficiency challenge; for example, hybrid backhaul networks including wireless and fiber links is a valuable solution. Moreover, the new sleeping model in small cells and adaptive power control of SBSs are effective approaches for saving energy in 5G wireless backhaul networks. However, more studies should be carried out in the near future.