Energy efficiency is among the most important network performance metrics, particularly when the end devices rely on limited energy sources, which is the case in many applications of the Internet of Things. In these cases, optimizing energy efficiency is tantamount to increasing the network lifetime. We focus on Low-Power Wide-Area Networks using the LoRaTM platform, and provide a thorough characterization of energy efficiency and device lifetime. Our formulations are used to demonstrate the effect of the network parameters on performance, and provide guidance on the appropriate parameter settings according to the application scenario. In addition to a complete analysis of the energy efficiency, we also calculate the device lifetime as a function of the transmission parameters, using a practical rule to estimate the channel usage per each device in the network, when complying to regulations and constraints to channel access due to fair access policies.