The energy demand of modern communities contributes significantly to climate change, increasing the release of greenhouse gases into the atmosphere. Energy efficiency is recognised as the key pathway to reducing energy usage while sustaining an equivalent, contemporary economic activity. In other words, to avoid climate change, mainstreaming energy efficiency finance is considered a top priority. This study focuses on introducing a rating system based on a Multi-Criteria Decision Analysis method that aims to promote the implementation and financing of energy efficiency investments. To this end, a benchmarking Tool is being deployed in order to materialise the proposed methodology and introduce a standardised procedure for benchmarking energy efficiency potential projects during the preliminary stages of investment conceptualisation. The proposed Tool exploits the Multi-Criteria Decision Analysis method ELECTRE Tri, taking into account major key performance indicators that are broadly used by investors and financing institutions to identify bankable energy efficiency investments and promote green transition. The methodology has been applied to benchmark 114 energy efficiency investments from eight different European countries. It should be mentioned that for the successful and effective development of the proposed Tool, input and feedback has been received by a variety of stakeholders from the energy sector and financing community, who also tested the Tool and confirmed that the approach proved to be extremely helpful to those seeking for sustainable investments in energy efficiency. The analysis resulted in the conclusion that the Tool covers the necessity for a standardised benchmark, providing added value to the energy efficiency market.