Energy efficient networks are becoming a hot research topic, and the networking community is increasingly devoting its attention to the identification of approaches to save energy in the networks of today. However, the networks of tomorrow will require built-in energy efficiency capabilities, so that new design techniques based on network models that account for energy efficiency are called for. One of the simplest approaches to obtain energy efficiency is based on the activation of network resources on demand, thus avoiding to always power on all the resources that are necessary to serve users during peak traffic periods. In this paper we both present a simple analytical model to determine the effectiveness of policies that activate APs (Access Points) in dense WLANs (Wireless LANs) according to the actual user demands, and quantify the performance that is achieved by such policies in terms of energy savings and QoS (Quality of Service). Numerical results show that, in the configurat...