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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Although renewable energy (excluding hydropower) is a relatively small portion of total energy supply both globally and in the United States, renewable energy installations in both the world and in the United States have nearly tripled between 2000 and 2008. Key Findings, continued • Worldwide, wind energy is the fastest growing renewable energy technology-between 2000 and 2008, wind energy generation worldwide increased by a factor of almost 7. The United States experienced even more dramatic growth, as installed wind energy capacity increased almost 10 times between 2000 and 2008. • In the United States, renewable energy has been capturing a growing percent of new capacity additions during the past few years. In 2008, renewable energy accounted for more than 43% of all new grid-connected electrical capacity installations in the United States-a large contrast from 2004 when all renewable energy captured only 2% of new capacity additions.

The primary objective of a power system is to safely provide reliable energy services to society at an affordable cost. In many countries, this objective has been supplemented by another, meeting energy demand with sustainable resources, which has culminated in the energy transition to low carbon and zero-carbon energy systems. This transition, occurring rapidly around the world, is characterized by the increasing penetration of variable renewable energy, inverter-based resources (IBR), and distributed energy resources (DER).While contributing towards sustainability goals, these trends can potentially compromise the primary objective of a power system given existing planning and operational paradigms. To continue to meet the primary objective as power systems transition, advances in the operation and planning of the power system are required. To meet this need, several system operators on the cutting edge of this transition have collaborated with technical institutions around the world to create the Global Power System Transformation (G-PST) Consortium. The consortium aims to support advanced system operator research, development activities, and all system operators globally with knowledge and training as they transform their power systems.