In the past few years, the Internet of Things is driving the need for extending the Internet to constrained devices, including sensors and actuators. The IPv6 Routing Protocol for Low power and lossy networks (RPL) is appearing as an emerging IETF standard especially tailored for Low Power Area Networks (6LoWPAN). RPL constructs a Direct Acyclic Graph (DAG) according to an objective function that governs the routing according to some metric(s) and constraint(s). In the last decade, several metrics and constraints have been proposed, but for the to best of our knowledge this is the first comparative evaluation of RPL energy-aware routing metrics. In this paper, we survey energy-aware routing metrics. Then, we evaluate the performance of each, considering grid and random topologies. Moreover, we consider in this evaluation two models for the exchange of messages: a model with no packet loss and a second one with 40% of packet loss. Our experiments show that multi-criteria metrics outperform other metrics.