Calorie requirement per a day of a child weighing 15 kg would be

We draw the energy from our food: The nutrients are transpoed to the body cells in the digestive tract after splitting up and are broken down there. This releases the required energy. They are measured in kilojoule (kJ) or kilocalories (kcal). One kilocalorie corresponds to 4,184 kilojoules. The calorie though is an outdated unit of measurement and was replaced in scientific use by the joule. In common parlance, calories are still often used. Reference: GHAI Essential pediatrics, 8th edition