ATP is a relatively unstable molecule because of the charges on the phosphate groups. As a result it's two high energy bonds and ester bond can be hydrolysed almost immediately to release two lots of 30.6 KJ and 14.2KJ respectively. Energy can exist in other forms such as glucose, glycogen and lipids which are longer term. Hence the relation of ATP to an energy currency where ATP is the cash and longer term forms are saving accounts etc.
