Cities are facing unprecedented challenges as the pace of urbanization, the last decades, has faced a significant increase. These challenges appear in several fields, such as supply, exchange, and consumption of energy. At the same time, the increasing demand for electrification by end-use sectors can lead to higher power fluctuations across the daily demand profile. In fact, it is well documented that the building sector demands approximately 40% of primary energy used within the European borders. Managing energy consumption is a multifaceted challenge; introducing grid flexibility and offering various innovative approaches for optimal use in both building and district level are the keys for a sustainable urbanization in the upcoming years. According to research, Decentralized Energy Systems (DES) using Renewable Energy Sources (RES) for energy production, offer an optimum solution for energy savings and grid flexibility, especially in stand-alone systems. However, due to the intermittent nature of RES and for meeting the load demand at any time, the need for energy storage systems is essential. Building thermal mass is a key parameter to mitigate inside temperature variations. Used with an optimized control strategy, a thermal mass increase is a solution to maintain a better thermal comfort, to stabilize heating and cooling loads and mitigate peak power demand. This study introduces both an efficient and flexible way to rate the thermal storage capability and through that the exploitation potential as a short-term energy storage system, by mainly using room temperature data as well as basic information of the building construction along with its Heating, Ventilation, and Air Conditioning (HVAC) equipment. So far, complex procedures are followed, which require extensive input of historical data, human efforts and time by developing theoretical models on simulation software, used by high experienced personnel. This study presents an empirical methodology for rating and exploiting the building thermal inertia in order to enhance RES penetration by evaluating its performance. The research is based on real data, harvested by an intelligent monitored building in Lavrion Technological and Cultural Park operated solely for research activities. The methodology will provide a tool for real time quantification and evaluation of building thermal mass, which could be integrated to intelligent control algorithms. The whole system through commercial monitoring technologies and Building Energy Management Systems (BEMS) will deliver to the market a low cost, reliable tool for efficient and precise control of the HVAC equipment aiming to maximize RES penetration without compromising occupants’ comfort levels.