"Separated by solar or wind" is sort of a red herring. If 1000 KW of solar energy is used to replace an equivalent amount of gasoline for cars, then other things that would have gotten that 1000 KW, say home lighting, would have to use energy from non-solar/wind generation instead. It isn't like there is a surplus of wind/solar energy generation. The argument is that energy can be produced more efficiently in generation plants rather than in combustion engines. The idea of mass produced hydrogen is that you are shfting the energy creation from the tank to the grid.