[citation needed] The basic model of energy budget may be shown as: P = C - R - U - F or P = C - (R + U + F) or
All the aspects of metabolism can be represented in energy units (e.g. joules (J);1 calorie = 4.2 kJ).
Ectotherms rely on the environment as a heat source while endotherms maintain their body temperature through the regulation of metabolic processes.
The heat produced in association with metabolic processes facilitates the active lifestyles of endotherms and their ability to travel far distances over a range of temperatures in the search for food.
The energy demands for ectotherms are generally one tenth of that required for endotherms.