Biogas plants utilizing organic waste play an important role in the transition toward a circular economy and renewable energy systems. However, evaluating their actual performance is challenging due to the diversity of anaerobic digestion technologies, the wide range of feedstocks, and the use of various pretreatment methods. Consequently, assessing operational efficiency requires a comprehensive approach that goes beyond installed capacity alone. This paper synthesizes and systematizes existing approaches for evaluating the efficiency of biogas plants based on key operational indicators reported in the literature. The analysis considers a broad spectrum of feedstocks, highlighting the variability of input materials and their influence on plant performance. Particular attention is given to the internal energy consumption of electricity and heat, which directly affects net energy output and overall efficiency. The relationship between annual energy production (MWh) and installed capacity (MW) is analyzed as a core performance indicator enabling comparison between plants using different technologies, substrates, and scales. The proposed framework supports transparent performance assessment, operational optimization, and evidence-based decision-making in the development and management of waste-based biogas systems.