Generating electricity from renewables varies greatly due to the unpredictable nature of the weather. In some cases, renewable power plants end up producing more electricity than is actually needed for current demands. Energy storage systems solve this problem by storing surplus energy and making it available at a later time as needed. Electricity can then be taken from the stored energy and fed into the grid.

Energy storage systems can integrate renewables by shifting energy to high-demand periods, or provide grid services like frequency control or spinning reserve. It's also possible to use the stored energy in the form of heat and cold, or as synthetic fuel e.g. for transportation. In addition to being a key component in the expansion of renewables and ensuring a sustainable, reliable and economic power supply, energy storage systems are also an important factor in what is known as "sector coupling".