Heterogeneous networks consisting of a macrocell tier and a small cell tier are considered an attractive solution to cope with the fierce increase of mobile traffic demand. Nevertheless, a massive deployment of small cell access points (SAPs) leads also to a considerable increase in energy consumption. Motivated by growing environmental awareness and the high price of energy, the design of energy efficient wireless systems for both macrocells and small cells becomes crucial. In this work, we analyze the trade-off between traffic offloading from the macrocell and the energy consumption of the small cell. Using tools from stochastic geometry, we define the user detection performance of the SAP and derive the small cell capacity accounting for the uncertainties associated with the random position of the user, the propagation channel, activity of the users, and the aggregate network interference. The proposed framework yields design guidelines for energy efficient small cells.