It has been observed that the correlation between ATP and biomass formation is very poor, and the observed growth yield is lowered under substrate-sufficient conditions. This indicates that the excess substrate causes uncoupling between anabolism and catabolism, which leads to the dissipation of non-growth energy. However, a quantitative description of such uncoupling remains elusive. Based on a balanced substrate reaction, a growth-yield model in relation to residual substrate concentration for substrate-sufficient continuous cultures was developed. On the basis of this yield model, a coefficient governing the uncoupling of anabolism and catabolism was defined. A model describing the effect of the residual substrate concentration on this uncoupling coefficient was further proposed. These models agree with experimental data very well. It is clearly shown that, under substrate-sufficient conditions, the variation in growth efficiency is mainly due to energy uncoupling rather than to maintenance energy expenditure.