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Successful operation of electric grid requires continuous real-time balancing of supply and demand including losses. The US grid operates effectively without much storage at present. However, the peak demand and generation facilities are growing at a faster rate in comparison to the transmission facilities. The system has observed some congestion problems. Over the past decade numerous reasons have accelerated the renewable energy applications in the existing grid. However, renewable resources are intermittent and the energy output varies widely during the day. Electricity storage technologies, when properly designed and integrated, can smooth out this variability and allow electricity to be dispatched at a later time. This paper concentrates on the energy storage technologies as a cost-effective technique that can be used in the US to make the grid more efficient, stable and reliable. Classification of energy storage technologies, comparison of various energy storage devices and system requirements are addressed in this paper.