Growth involves two flows of energy: the chemical energy in the monomers used to construct the macromolecules that comprise tissue (proteins, nucleic acids, lipid membranes), and the metabolic energy used to build those macromolecules. The metabolic costs of synthesising the macromolecules necessary to build tissue are well defined, and we have a robust estimate of the overall cost of growth for an individual ectotherm. At the population level the cost of production appears to be much greater for endotherms than ectotherms, the reasons for which are not fully understood. These uncertainties are important to resolve if we wish to accurately model the flow of energy through populations or ecosystems because simply scaling up from individual energetics may produce misleading results.