ATP is best suited for short-term energy storage because it is too unstable for long-term storage. When cells need to store chemical energy in a more stable form, they use the energy from ATP to build more stable molecules. The short-term excess energy from ATP is beneficial to build starch/sugar and fat molecules with the help of an outside source in some instances.
Lipids are organic molecules which can be used for long-term energy. These includes fats, waxes, sterols, glycerides, phospholipids, etc. Lipids contain more energy per unit mass which makes it much lighter. However, their insolubility to water makes them harder to transport.
