Energy storage is a suite of technologies that capture and store energy for use at a later higher value time. Energy can be stored in many ways: chemically (e.g. batteries), gravitationally (e.g. pumped hydro, pictured here), mechanically (e.g. flywheels) and thermally (e.g. molten salt). On the electric grid, where supply must always equal demand, energy storage increases efficiency by avoiding expensive transmission and distribution upgrades and substituting for the most polluting “peaker” power plants. Energy storage is also expected to play a key role in enabling the grid to integrate large quantities of variable renewable energy resources such as solar and wind.