  The resonant tunneling model is the simplest model for describing electronic
transport through nanoscale objects like individual molecules. A complete
understanding includes not only charge transport but also thermal transport and
their intricate interplay. Key linear response observables are the electrical
conductance G and the Seebeck coefficient S. Here we present experiments on
unspecified resonant tunnel junctions and molecular junctions that uncover
correlations between $G$ and $S$, in particular rigid boundaries for $S(G)$. We
find that these correlations can be consistently understood by the single-level
resonant tunneling model, with excellent match to experiments. In this
framework, measuring $I(V)$ and $S$ for a given junction provides access to the
full thermoelectric characterization of the electronic system. A remarkable
result is that without targeted chemical design, molecular junctions can expose
thermoelectric conversion efficiencies which are close to the Carnot limit.
This insight allows to provide design rules for optimized thermoelectric
efficiency.
