This refers to newer PSH installations and older PSH systems may have efficiencies closer to the 60-75% range. **: As CAES relies on both electricity to compress air and a fuel (typically natural gas) to expand the air, its efficiency cannot be readily compared to other storage technologies. The value used in this report represents the ratio of the output of electrical energy to the combined input of electrical energy for the compressor and the natural gas input for expansion, using the heating value of natural gas to convert its energy to how much electricity it could have produced (Mongird et al. 2019). † This range refers to a 10 MW 4-hour battery in 2020 costs. For lithium-ion, this refers to the NMC chemistry (see Section 2.1 for additional information on lithium-ion chemistries). See Mongird et. al. (2020) for additional energy storage sizes and durations and estimates for future years. † † : This range refers to 2018 costs. See Mongird et. al. (2019) for future years. † † † This range refers to 1000 MW 10-hour systems. See Mongird et. al. (2020) for additional energy storage sizes and durations and estimates for future years. † † † † This range refers to 100 MW 10-hour systems. See Mongird et. al. (2020) for additional energy storage sizes and durations and estimates for future years.

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