The algebraic difference between the average neutron lifetime (l) and the average generation time (g), referred to as the excess time E (= l − g), is shown to be a useful parameter giving physical insight into the degree of utilization of neutrons toward a chain reacting process in a complicated fissionable system. It can be used to support physical arguments in checking the validity of complex computer results as well as to give some rationale as to what results to expect in these calculations. The concept is applied to the classic criticality case of dry or wet storage of separated fuel assemblies in a variable density hydrogenous moderator.