Thermal energy storage (TES) is recognised as a key technology for further deployment of renewable energy and to increase energy efficiency in our systems. Several technology roadmaps include this technology in their portfolio to achieve such objectives. In this paper, a first attempt to collect, organise and classify key performance indicators (KPI) used for TES is presented. Up to now, only KPI for TES in solar power plants (CSP) and in buildings can be found. The listed KPI are quantified in the literature and compared in this paper. This paper shows that TES can only be implemented by policy makers if more KPI are identified for more applications. Moreover, close monitoring of the achievements of the already identified KPI needs to be carried out to demonstrate the potential of TES. ; The work is partially funded by the Spanish Government (ENE2011-28269-C03-02, ENE2011-22722). The authors would like to thank the Catalan Government for the quality accreditation given to their research group GREA (2014 SGR 123) and research group DIOPMA (2014 SGR 1543). The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) under grant agreement n° PIRSES-GA-2013-610692 (INNOSTORAGE).