Question

When a cell has energy available, how can it store small amounts of that energy?

Answer 1

Cells manage to perform a wide range of functions in such a small package- growing, moving, housekeeping, and various other activities and most of those functions are energy dependent.

Complete answer:
When energy is available to the cell, it can store very small amounts of energy by addition of a phosphate group to ADP molecules forming ATP molecules. The energy stored as ATP is then released when the ATP is converted back to ADP(Adenosine diphosphate) and a phosphate group.
Cells do not store energy, instead they produce it upon need.They break down glucose molecules and make ATP which is then used for the needed activity. Excess glucose can be stored by the cell but not the energy. The glucose is stored as glucose and the excess of it is turned into the glycogen. If more and more of the glucose is coming to the body with the intake of food, then this extra glucose needs to be turned into lipid and then stored as such.
When energy is abundantly available, the eukaryotic cells tend to make larger, and energy-rich molecules to store some of their excess energy. Therefore, the resulting sugars and fats — or in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are quite large enough to be visible in electron micrographs.

Note: Cells do need energy to accomplish the certain tasks of life. Beginning with the energy sources obtained from their environment majorly in the form of sunlight and the organic food molecules, eukaryotic cells produce energy-rich molecules such as ATP and NADH via some energy pathways including photosynthesis, glycolysis, the citric acid cycle, and the oxidative phosphorylation. Any excess of energy is then further stored in larger, energy-rich molecules like polysaccharides (i.e. starch and glycogen) and lipids.