Monolayers of colloidally synthesized cobalt-platinum nanoparticles of different diameters characterized by transmission electron microscopy were deposited on structured silicon oxide substrates and characterized by scanning electron microscopy, grazing incidence X-ray scattering, and electric transport measurements. The highly ordered nanoparticle films show a thermally activated electron hopping between spatially adjacent particles at room temperature and Coulomb blockade at low temperatures. We present a novel approach to experimentally determine the particles charging energies giving values of 6.7-25.4 meV dependent on the particles size and independent of the interparticle distance. These observations are supported by finite element method calculations showing the self-capacitance to be the determining value which only depends on the permittivity constant of the surrounding space and the particles radius.