We investigate key electrostatic features of charged dendrimers at hand of the biomedically important dendritic polyglycerol sulfate (dPGS) macromolecule using multiscale computer simulations and Zetasizer experiments. In our simulation study, we first develop an effective mesoscale Hamiltonian specific to dPGS based on input from all-atom, explicit-water simulations of dPGS of low generation. Employing this, in coarse-grained, implicitsolvent/explicit-salt Langevin dynamics simulations, we then study dPGS structural and electrostatic properties up to the sixth generation. By systematically mapping then the calculated electrostatic potential onto the Debye-Huckel form that serves as: a basic defining equation for the effective charge we determine well-defined effective net charges and corresponding radii, surface charge densities, and surface potentials of dPGS. The latter are found to be up to 1 order of magnitude smaller than the bare values and consistent with previously derived theories on charge renormalization and weak saturation for high dendrimer generations (charges). Finally, we find that the surface potentials of the dendrimers estimated from the simulations compare very well with our new electrophoretic experiments.