The cost of generating electricity from solar and wind has fallen far enough that it is the cheapest new generation in most of the world. That success created the actual problem: output does not match demand.
The shape of the mismatch
Solar peaks in the middle of the day. Demand peaks in the early evening. The gap is a few hours, every day, predictably.
That is the easy version. The hard version is seasonal – regions where winter demand is high and winter generation is low, with a gap measured in months rather than hours.
What batteries solve
The daily gap, well. Lithium batteries at grid scale have been deployed rapidly, are economically viable without subsidy in several markets, and shift energy by hours very effectively.
They do not solve the seasonal problem, because storing energy for months in batteries is prohibitively expensive.
The alternatives for longer durations
Pumped hydro, which is mature, cheap per unit stored, and requires specific geography that mostly already has it.
Flow batteries, which scale duration independently of power by simply using bigger tanks. Lower energy density, unlimited cycle life, and well suited to long-duration storage.
Thermal storage – heating a material and recovering the energy later. Cheap materials, moderate efficiency, promising for industrial heat.
And hydrogen, which is inefficient and is the only option that stores across seasons.
What actually gets built
Batteries, overwhelmingly, because the daily arbitrage pays now. Long-duration storage is needed at higher renewable penetration and does not pay yet, which is a classic timing problem.
The underrated part
Demand flexibility. Shifting when electricity is used – vehicle charging, water heating, industrial processes – is cheaper than storing it. Every unit of demand moved is a unit of storage not built.