Calculations were made of the magnitude of the wind force for self-electromigration of adatoms on heterogeneous metal surfaces. By using a layer-KKR method, the wind valence of adatoms within small monolayer islands and voids on Cu(100) was calculated. It was shown that the electromigration driving-force felt by an adatom was significantly altered by the scattering of carriers among neighboring atoms within the island. By using the results of these calculations as the input for kinetic Monte Carlo simulations, it was shown how this inhomogeneity of the wind force was reflected in the driven surface diffusion of Cu islands on a current-carrying Cu(100) substrate.