EU's energy sector is facing turbulent times as it strives to strengthen energy independence without losing sight of providing affordable and sustainable energy to all. The European Commission's REPowerEU plan to accelerate the EU's energy transition places additional pressure on each member state's path to energy sustainability. To reach this objective, policymakers must assess the present energy sustainability levels of each member state, identify areas for development, and monitor the county's progress over time. The purpose of this article is to analyze and compare the energy sustainability levels of the EU member states using a variety of indicators and to identify key cornerstones for advancing their energy transition. This study develops an energy sustainability composite index (ESCI) in order to unravel and compare the multiple lawyers of energy sustainability, including energy security, primary energy intensity, share of renewable energy resources, energy efficiency, CO2 emission intensity, and energy poverty. Log-Mean Divisia Index (LMDI) decomposition analysis is utilized to track the progress of energy policy in achieving reductions in energy-related CO2 emissions from 2015 to 2019. Changes in CO2 emissions were decomposed using Kaya identity factors to determine which of the following factors contributed the most to the changes: changes in emission intensity, energy intensity, economic or population growth. The results indicate that all EU member states have untapped potential for improving energy sustainability.