Energy storage systems are essential for storing excess energy from intermittent renewable sources like wind and solar. There are several technologies available for energy storage, including pumped storage hydroelectricity, batteries, flywheels, compressed air, and regenerative fuel cells. Each technology has its strengths and weaknesses related to factors like energy capacity, output power, lifetime, and costs. Pumped storage hydro can store large amounts of energy but is location dependent, while batteries provide flexible output but have lower energy capacity than other options. Energy storage allows for balancing energy supply and demand and improving grid reliability when integrating renewable energy.