How does ADP store energy?

ADP stores energy in the bonds joining the phosphate groups.

ADP stands for adenosine diphosphate. It is a nucleoside phosphate comprised of one adenine molecule, a ribose sugar and two phosphate groups bonded in series to the 5′ carbon of the pentose sugar via high-energy bonds. These bonds are where the ADP stores energy. When energy is needed, ADP cleaves its second phosphate group, releasing energy stored within the bond.