Expression of bacterial cellular processes (maintenance, heat loss, growth, cometabolism and substrate degradation) into energy units yielded surprisingly realistic figures on the bacterial energy balance in spite of the severe approximations and assumptions that had to be made. For studies without cometabolism, 47-83% of the calculated amount of available energy was consumed by growth; for maintenance + heat loss this percentage was 20-35%. When involved, cometabolism consumed 7-13% of the total energy budget. Overall, 67-118% of the calculated amount of energy generated was spent on these energy-consuming processes. This shows that the model is internally consistent. Relationships between growth and cometabolism may offer predictions of persistence of cometabolizable chemicals under different conditions. The energy budget model as presented provides a starting point for the development of such relationships. In addition, the method is used to explain threshold concentrations, below which normally degradable compounds are not degraded.