The method of energy storage may be classified according to the form in which energy is being stored.

A) Mechanical energy storage
1) Pumped hydroelectric storage
2) Compressed air
3) Flywheel

B) Electrical Energy storage ...the Lead acid storage

C) Chemical energy storage
1) Hydrogen
2) Ammonia
3) Reversible chemical reactions

Chemical energy storage methods are widely used in fuel cells

D) Electromagnetic Energy storage

E) Thermal energy storage
Sensible heat
Latent heat
Chemical reactions

F) Biological Storage