This paper analyzes the energy consumption of ad hoc nodes using IEEE 802.11 interfaces. Our objective is to provide theoretical limits on the lifetime gains that can be achieved by different power saving techniques proposed in the literature. The evaluation takes into account the properties of the medium access protocol and the process of forwarding packets in ad hoc mode. The key point is to determine the node lifetime based on its average power consumption. The average power consumption is estimated considering how long the node remains sleeping, idle, receiving, or transmitting.