An energy budget is a balance sheet of energy income against expenditure. It is studied in the field of Energetics which deals with the study of energy transfer and transformation from one form to another. Calorie is the basic unit of measurement. An organism in a laboratory experiment is an open thermodynamic system, exchanging energy with its surroundings in three ways - heat, work and the potential energy of biochemical compounds.

Organisms use ingested food resources (C=consumption) as building blocks in the synthesis of tissues (P=production) and as fuel in the metabolic process that power this synthesis and other physiological processes (R=respiratory loss). Some of the resources are lost as waste products (F=faecal loss, U=urinary loss). All these aspects of metabolism can be represented in energy units. The basic model of energy budget may be shown as:

P = C - R - U - F or

P = C - (R + U + F) or

C = P + R + U + F

All the aspects of metabolism can be represented in energy units (e.g. joules (J);1 calorie = 4.2 kJ). Energy used for metabolism will be

R = C - (F + U + P)

Energy used in the maintenance will be

R + F + U = C - P