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Assembly maintained or transiently created by consumption of a chemical fuel. A fuel-triggered state can decay as the fuel is irreversibly converted to waste, even though rapid binding equilibria hold within each instantaneous composition.

Storage has multiple performance measures: energy and power density, efficiency, cycle/calendar life, duration, cost, materials, safety and siting. Lithium-ion batteries offer high round-trip efficiency, fast response and compactness, making them useful for vehicles and short-duration grid services. That does not make them optimal for every duty. Pumped hydro can suit large stationary energy stores where geography permits; thermal stores can serve heat directly; chemical fuels or other long-duration systems can trade efficiency for cheap energy capacity. A technology that is excellent per kilogram may be poor for seasonal storage. “Best” requires a specified service and life-cycle comparison.