Different from other studies in 5G networks, we focus on the throughput and energy efficiency of 5G wireless backhaul networks considering ultra dense small cells and millimeter wave communications. In detail, we first configure two typical small cell scenarios for comparison analysis. Then, we evaluate the wireless backhaul traffic models based on two typical small cell scenarios with different spectrum efficiencies. Furthermore, the energy efficiency of 5G wireless backhaul networks in two typical scenarios is analyzed by numerical results. Moreover, the impact of different frequency bands of wireless backhaul links on the energy efficiency of 5G backhaul networks is investigated for two typical scenarios. Finally, future challenges of 5G wireless backhaul networks are discussed, and conclusions are drawn.