Energy can be stored as heat, kinetic energy, potential energy, chemical energy and electrical energy.

These types of energy storage are shown in the figure below, in terms of the amounts stored and the power with which the storage can be discharged.

The largest amount of energy can be stored as chemical energy, such as in natural gas storage facilities. Just as important, however, is how quickly a storage facility is available for discharge. Here, electrical storage methods, such as in batteries or capacitors, are the fastest. They can be discharged without any significant time delay. Pumped-storage plants and compressed-air storage are ready within a few minutes, while power plants powered by natural gas, for example, require a lead time in the range of hours. 

Different options for electricity storage  
after Sterner. M., Stadler. I.: "Energiespeicher", Springer Vieweg (2014)

Pumped storage and compressed air storage can store energy for long periods of time in the range of GWh.