How much more energy does fat supply than carbohydrates?

Fats actually contain a lot more energy than carbohydrates. The reason carbohydrates are used for energy before fats is because carbohydrates are easier to break down; reactions involving carbohydrates generally have a lower activation energy. This means that less energy is used to release the energy in the bonds of the carbohydrates than fats. In other Words, although fats contain more energy, it is more efficient for the body to break down carbohydrates than fats. Once carbohydrate supplies diminish, fat stores can be broken down to use for energy, but doing so is not the body's favorite thing to do.