E = mc²
E = mc² means that the energy released by the destruction of mass is the mass destroyed multiplied by the speed of light (c) squared.
In real terms, this means that if 50g of mass, which is about the weight of a chocolate bar, were transformed into energy, you could run Princeton University's campus (12MW average consumption) for an entire year!
In contrast, if you eat that chocolate bar and burn it chemically in your system, all you might be good for is a short jog.