Your body can convert somewhere in the range of 0.8%-1% of your stored fat into usable energy per day. If you have 150lbs of fat on you, you can covert about 1.5lbs of that into energy. Since fat has approximately 3500 Calories per lbs, that's 5250 Calories available to you without eating. If you're not that much of a fat ass and have 30 lbs of fat, you can only get 1050 Calories from it per day. In that case, you need to eat or the body will start breaking down other things such a muscle and your energy level will crash.
This all ignores the various energy buffers your body has such as glycogen but run out quickly if you're fasting.
