How big is the Flash's grocery bill?
For this superheroic Fermi Question you'd might like to know that every Calorie that you eat has about 4200 Joules of energy. Also guessing how fast he goes is tough. Consider average days for the Flash and think to yourself that he's much faster than the speed of sound but not any where near as fast as the speed of light. (Just an average day. That's all I'm saying.)