Energy expenditure refers to the amount of energy that is used or consumed by an individual, group or system over a period of time. This can be measured in various ways such as calories burned, joules used or watt-hours consumed. Energy expenditure is important because it helps determine how much energy someone needs to consume in order to maintain their body weight and overall health. It also plays a role in determining an individual's metabolic rate which can affect their ability to lose or gain weight. In addition, energy expenditure can be used as a measure of physical activity levels and is often used in the study of exercise physiology and sports science.