Energy storage systems provide several benefits and services in optimizing the power grid's reliability, efficiencyand safety. However, the feasibility of energy storage systems varies dependent on the requirements of theapplications. Technical limitations in design and type of the storage technology prevents a single storage type toperform equally well in all situations. Hence, it is essential to compare and measure the energy storages’ usefulnessand determine their optimal use. To address this issue this study introduces an indexing approach toevaluate the suitability of energy storages for power system applications. Four different energy storages (doublelayercapacitor, flywheel, lead–acid battery, lithium-ion battery) are tested for four typical energy storage applications(frequency regulation, voltage support, capacity firming and energy time-shift). The suitability indexallows a simple and intuitive way to compare and rank the suitability of energy storages.