We study in this paper the impact of small cells on the capacity and energy consumption of an LTE-Advanced network in a heterogeneous setting where macro cells coexist with outdoor pico and femtocells. We analyze the Erlang-like capacity in such an environment and show the gains obtained from offloading traffic from macro cells to small ones. Energy efficiency is used throughout this study, bringing together network performance and its overall power consumption. Our results show that small cells are a good alternative to network densification as they achieve higher network capacities with good energy efficiency. In order to further improve the efficiency of the resulting heterogeneous network, we propose a sleep mode that switches off some picocells when the traffic is low and quantify the resulting increase in efficiency.