Sustainable energy systems in the EU abound, but how cost efficient each one really is has been a matter of debate for researchers, stakeholders and policymakers. The EU-funded project 'Cost assessment for sustainable energy systems' (CASES) has taken up the challenge to evaluate their cost efficiency. It looked at the social cost of energy and disseminated this valuable knowledge to concerned stakeholders. To achieve this, the project built a comprehensive assessment of external and private costs for different electricity generation technologies in EU countries. In addition, it evaluated policy options for improving the efficiency of energy use. CASES sought to establish comparable value terms to select among the alternatives, considering two different valuation approaches. The first involves neo-classical economics and provides value information needed in cost-benefit analyses, while the second uses Multiple Criteria Decision Analysis (MCDA) which compares several actions along multiple competing criteria. The project also investigated several weighing techniques to isolate the most ideal ones for effective evaluation and examined the different components of a policy appraisal procedure. It then looked at the post-assessment phase and how best to transform the results of the appraisal procedure into a viable policy plan. An important set of recommendations for assessing sustainable energy systems has been produced, an accomplishment that will undoubtedly upgrade energy solutions for the EU.