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Energy storage helps overcome barriers for intermittent renewable energy and is an important aspect of a sustainable energy system. The most commonly used storage method is pumped-storage hydroelectricity, which requires locations with large differences in height and access to water. Batteries, and specifically lithium-ion batteries whose costs have been coming down rapidly, are also deployed widely. Batteries typically store electricity for short periods; research is ongoing into technology with sufficient capacity to last through seasons. Pumped hydro storage and power-to-gas (converting electricity to gas, and back) with capacity for multi-month usage has been implemented in some locations.