Intermittent nature and variable power levels of renewable energy sources represent major obstacles to their integration within the existing conventional electrical power grid that requires uninterrupted (highly reliable) and smooth power delivery. In order to improve both the reliability of renewable energy systems and their on-demand power availability, various storage systems such as those based on flywheels, compressed air, batteries and ultracapacitors have been considered in literature and applications. The main aim of this work is to review and compare the aforementioned energy storage systems in terms of efficiency, capital costs, energy/power capacity, and reliability. Some aspects of storage system dynamics modeling related to energy storage monitoring (i.e. state-of-charge monitoring) are outlined as well.