As the ground-based wind-turbine systems have steadily reached their performance peak due to turbine blade size limitations, generator size constraints, high investment costs, and relatively unpredictable nature of near-surface winds, the possibility of harnessing the energy of steady, high-altitude/high-speed winds has become increasingly attractive within the last decade. However, due to the intermittent nature of airborne wind energy systems power production, sufficiently large energy storage is needed in order to provide constant power supply to the electrical power grid. This paper focuses on the sizing of typical low-to-medium scale energy storage systems (up to 10 MW), including those based on flywheels, compressed air, batteries and ultracapacitors, considering the intermittent power production cycle, airborne module altitude range and ground-station generator power ratings. The assessment results are summarized in terms of investment/running costs, storage system size, and durability, thus providing practical guidelines for the choice of appropriate energy storage system.