The increasing penetration of renewable energies into the energy system is leading to significant development and deployment of generation plants based on the use of solar energy. On large scale, concentrating solar power (CSP) plants is the starring technology. In those systems, thermal energy storage (TES) is an essential component that allows both providing dispatchability and increasing power production thus improving the plant efficiency and reducing both its size and cost. Current storage technologies are mainly based on solar salts (molten salts) and saturated steam, their main drawback being their operating temperatures. Therefore, the current challenge is to develop innovative solutions that allow CSP plants to work at higher temperatures to increase their efficiency. In order to allow the comparison of new TES concepts, the definition of proper key performance indicators (KPIs) helps to better identify the best storage solutions. This paper reports a preliminary selection of KPIs suitable for TES systems and their validation against the commercially available two-tanks molten salts storage concept for a 100 MW net capacity CSP tower plant to allow the comparison with innovative solutions. The results of this study can be used as a basis for comparison of TES technologies with the available ones, as well as to set the target for the research and development of future storage solutions.