Energy requirements are the number of calories needed to sustain a person leading a particular life — the daily energy intake that maintains weight and health. They are built from basal metabolic rate plus activity: a sedentary office worker needs far less than a manual labourer or an endurance athlete, and requirements shift with age, sex, body size, pregnancy, lactation, illness, and climate. Authorities publish reference values by age and activity band, and dietitians use them to plan diets, set rations, and judge whether reported intakes are plausible. The numbers are averages with wide individual variation — two people of the same size can differ by hundreds of calories. Energy requirements are the demand side of the energy-balance equation, the target that dietary intake must meet without overshooting.