  Increasing experimental evidence is building which indicates that signatures
of strong correlations are present in the Na rich region of Na$_x$CoO$_2$ (ie.
$x\approx0.7$) and absent in the Na poor region (ie. $x\approx0.3$). This is
unexpected given that NaCoO$_2$ is a band insulator and CoO$_2$ has an integer
filled open shell making it a candidate for strong correlations. We explain
these experimental observations by presenting a minimal low-energy Hamiltonian
for the cobaltates and solving it within LDA+DMFT. The Na potential is shown to
be a key element in understanding correlations in this material. Furthermore,
LDA calculations for the realistic Na ordering predict a \emph{binary}
perturbation of the Co sites which correlates with the Na$_1$ sites (ie. Na
sites above/below Co sites).
