Understanding the energy consumption of wireless interfaces is critical to provide guidelines for the design and implementation of new protocols or interfaces. In this work we analyze the energy performance of an IEEE 802.11 WLAN. Our contributions are as follows: i) we present an accurate analytical model that is able to predict the energy consumption, ii) we present an approximate model that sacrifices accuracy for the sake of analytical tractability, iii) based on this simplified analysis, we derive the optimal configuration to maximize energy efficiency of a WLAN, and iv) finally, we also analyze the tradeoff between throughput and energy efficiency that IEEE 802.11 imposes.