Everybody assumes batteries (lithium etc.) as the way to go. However the greenest / most efficient way to store the energy is actually just pumping water uphill to a reservoir, and extracting the energy by letting it flow back down.
Of course this storage system only works wherever there is someplace "higher" to pump the water. My understanding is that it's not worth the trouble to build a hill/mountain to pump the water up.
And most already build hydroelectricity facilities can be retrofitted (although at high cost) to become pumped-storage hydroelectricity facilities.
Really? You need a huge downstream reservoir to hold the water that you are going to pump to the upstream reservoir.
What does green mean here? Most of the developed world's easily used hydro sites are already in use. Expanding them in most countries means relocating lots of people, building huge dams, drowning towns, villages, and historical artefacts, losing agricultural land, disturbing river flows and irrigation systems.
Not always very 'green'.