Daily food intake and spent energy should be equal. It is rather necessary that the intake is less than the spent energy.
If the food we eat provides more energy than our body can spend, the body reacts in a way that it retains the part which is not needed and it turns into fat. And the consequences are well known.
How to determine the ideal weight?
The ideal weight is the one that will remain after you subtract the number 10 out of your height. So the ideal weight for a person who is 165 cm in height is 55 kg / 121 pound. However, all this is relative because muscle and bone mass are individual. According to that, the result may vary from two to three pounds. Persons who are not physically active should reduced their daily food intake somewhere between 1200 and 1500 calories, while individuals who do sports are allowed to intake 2000 calories.