Electron transport in a photoelectrode consisting of a porous nanostructured semiconducting or insulating network interpenetrated with an electrolyte solution is considered. Electrons, photogenerated in the solid network by light incident from the electrolyte side, travel through the system and are removed at the substrate/network boundary. We calculate the driving force for electron diffusion through the network from first principles. It is found that the driving force is in the order of kT/e divided by the thickness of the network and independent of the light intensity. Photoinduced interfacial charging of the network due to electron trapping can enhance the driving force but does not change the transport characteristics.