Some may take the chemistry of rocks for granite but I draw the line at ATP. By ATP, I mean adenosine triphosphate. Biochemists call ATP the "molecular currency unit" of cellular energy. Every currency is denominated in units. ATP may be the gold standard of molecular biology, but calories make as much sense as a currency unit because we all know what a surplus and deficit of calories are. As an aside, what we call calories are actually a thousand "little" calories. I guess we're too stupid--or too frightened by the actual numbers of calories in a "Calorie" of food to give the unit it's proper name -- a kilocalorie. So if you sit down and eat a 1,000 calorie meal, you're actually eating a 1000 x 1000 calories or a million calorie's worth of food. A thousand calorie meal is actual a Megacalorie meal. Does that have marketing potential?
Caloric energy is stored in phosphorus oxygen bonds by drying them. When phosphoric acid is dried, it expels water from its core and links up with another phosphate to make what are called anhydrides. That takes energy and chemotrophic cells consume the chemical energy stored in foods to do this drying. Fortunately, anhydride chains produced for energy storage like ATP are meta-stable, even in the presence of water, and need enzymes called kinases or phosphatases to come apart again. That's when the caloric energy payback occurs.