Nowadays, cities are key to meet sustainability objectives in the European Union. Almost 75% of European habitants live in cities and the majority of cities authorities have committed to increase energy efficiency by improving buildings, equipment and facilities' performance. Actually, about 35% of the EU's buildings are over 50 years old and by improving the energy efficiency of buildings, the total EU energy consumption can be reduced by 5% to 6% and lower CO2 emissions by about 5%. In this context, the main aim of this paper is to present an online web tool(1), for improving the energy performance of existing buildings in Greece based on the actual energy use. The application is able to collect real energy consumption data and provide, with a user friendly way, a toolset which calculates energy savings from a list of improvement scenarios. The online software calculates automatically the economic viability of the chosen actions. The web tool has been tested in different buildings in Greece and it is developed using open source software and programming languages such as PHP and MySQL. The application has been developed by the Decision Support Systems Laboratory (DSS), of the School of Electrical & Computer Engineering, National Technical University of Athens (NTUA). The web tool has been used from the students of NTUA for the academic year 2014-2015, resulting in a number of completed case studies. The main outcomes from the 1st year of the web tool's real life application are summarised in the paper, offering useful results about the energy building capacity in the capital of Greece.
