Narrowband Internet of Things (NB-IoT) has quickly become a leading technology in the deployment of IoT systems and services, thanks to its appealing features in terms of coverage and energy efficiency, as well as compatibility with existing mobile networks. NB-IoT devices are expected to have a battery life of 5-10 years, however, such a long battery lifetime can only be achieved provided the NB-IoT device is configured with energy efficiency in mind. In this talk, we will focus on the energy consumption of NB-IoT devices. We will first highlight the different power saving modes defined in NB-IoT. We will then investigate how different configurations can impact energy consumption through real-world measurements as well as extensive simulations. Based on these observations, we will provide guidelines on the device configuration that can lead to minimal energy consumption.