Energy storage systems provide several benefits and services in optimizing the power grid's reliability, efficiency and safety. However, the feasibility of energy storage systems varies dependent on the requirements of the applications. Technical limitations in design and type of the storage technology prevents a single storage type to perform equally well in all situations. Hence, it is essential to compare and measure the energy storages’ usefulness and determine their optimal use. To address this issue this study introduces an indexing approach to evaluate the suitability of energy storages for power system applications. Four different energy storages (double-layer capacitor, 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 index allows a simple and intuitive way to compare and rank the suitability of energy storages.