Small cells and heterogeneous network approaches in mobile networks are very promising as they provide an enhancement of capacity without a significant impact on the energy consumption when properly designed. Yet, recent studies have pointed out that the backhaul energy consumption cannot be neglected when a small cell deployment is introduced. In this paper, we provide a comprehensive study on energy efficiency, cost issues and trade-offs for backhaul infrastructure when the influence of Radio Access Network (RAN) design and growth strategies are considered. For the case study, we use two well-known backhaul architectures, one supported on fiber-to-the-building (FTTB)+ 10Gbps passive optical Network (10GPON) and a second which consists of FTTB+ Microwave backhauling. The numerical results and analysis performed provide significant insights of the potential energy savings, CAPEX, OPEX and challenges for backhaul and RAN architecture design.