We present a study case comparing conventional single-antenna (SISO) to multiple antenna (MIMO) WLAN systems in terms of capacity and energy efficiency. The analyzed scenarios are two real buildings and a Monte Carlo simulation. We found that, in the real scenario, energy efficiency approximates 10 Mbps/W with SISO systems, while multi-user (MU)-MIMO approximates 30 Mbps/W with single-antenna STAs and 60 Mbps/W with two-antenna STAs. Monte Carlo simulations show that the energy efficiency with MU-MIMO can achieve up to 88 Mbps/W approximately. We also discuss the impact of several radio parameters and network design on the energy efficiency of WLANs.