Comment by BobaFloutist

Right, but a jar of peanut butter is more comparable to a gas can than to a solid-stste battery. I'm sure if you measured 100% of the kcal comprising the battery in a bomb calorimeter, the source of measurements of the kcal of food, you'd end up with a significantly higher density - but wholesale combustion is the paradigm we're trying to move past, not towards.