We measure the energy our bodies use in calories. In general, a calorie from one source has exactly the same energy as a calorie from another.
Every person has different calorie needs.
Although Nutrition Wizard provides an estimate of your daily calorie usage, it is only a best-guess. Whatever regular calorie intake results in a stable weight for you is what your body actually uses. You lose weight when you use more calories than you take in and you body uses up stored fat. If you eat more calories than you burn, you gain weight. One pound of fat contains about 3500 calories.