Energy efficiency is a critical metric for the sustainability and performance of Narrow Band Internet of Things (NB-IoT) networks. Despite the burgeoning research in this area, there exists a need for a comprehensive review to synthesize the current methodologies and findings. This study aims to systematically review the literature on evaluating energy efficiency in NB-IoT networks, providing a consolidated view of the state-of-the-art methods and outcomes. We adopted the PRISMA 2020 protocol for the selection and evaluation of papers. A total of 94 articles from recognized databases were screened, 77 of which met the inclusion criteria. Additionally, thematic analysis was employed to identify key themes and a word cloud was generated to visualize frequently occurring terms. The study offers a comprehensive overview of methodologies and findings related to energy efficiency in NB-IoT networks. It highlights the need for standardization in evaluation metrics and points out emerging areas for future research. Researchers and practitioners interested in the state of energy efficiency in NB-IoT systems will find this review to be an invaluable resource.