In this paper, we describe a major issue of ad-hoc networks. It concerns energy consumption since nodes are usually mobile and battery-operated. We present the results of a detailed simulation comparing, with respect to energy consumption, four ad hoc routing protocols that cover a range of design choices: DSR, AODV, DSDV and a new protocol OLSR [3]. A set of experiments was carried out to evaluate how the different approaches and algorithms affect the energy usage in mobile devices.