  In this paper we emphasize the close connection between the chemical and
spectrophotometric evolution of stellar systems: Chemical yields from stars
correspond to a precise fraction of their emitted light. We translate this
concept quantitatively. Starting from simple stellar populations, we derive
useful analytical relations to calculate the stellar fuel consumption (emitted
light) as a function of basic quantities predicted by stellar models, i.e. the
mass of the core and the chemical composition of the envelope. The final
formulas explicate the relation between integrated light contribution (total or
limited to particular evolutionary phases), chemical yields and stellar
remnants. We test their accuracy in the case of low- and intermediate-mass
stars, and indicate the way to extend the analysis to massive stars. This
formalism provides an easy tool to check the internal consistency between the
different stellar inputs adopted in galaxy models: The fuel computed by means
of the analytical formulas (corresponding to a given set of chemical yields)
should be compared to the exact values given by the luminosity integration
along the stellar evolutionary tracks or isochrones (corresponding to a given
set of spectrophotometric models). Only if both estimates of the fuel are
similar, the stellar inputs can be considered self-consistent in terms of their
energetics. This sets an important requirement to galaxy models, also in
consideration of the fact that different sources of input stellar data are
frequently used to model their spectro-photometric and chemical evolution.
