Several possible definitions for energetic growth efficiency are reviewed. Although definitions based on a simple comparison of inflowing and outflowing energy suffer from shortcomings, they are the only ones that can be related to the respective energy dissipation in a unique way, even in cases with complex growth stoichiometry. It is therefore possible to determine the enthalpic growth efficiency in a very simple way by calorimetry. In completely aerobic growth, the result is also virtually equal to the free energy efficiency. In mixed respiro-fermentative growth, the two efficiencies are different and the free energy efficiency must be computed based on detailed knowledge of the growth stoichiometry. In aerobic, mixed, and anaerobic growth of Kluyveromyces fragilis yeast, the free energy efficiency remained constant at about 60%.