Direct photoelectrochemical separation of water into Hydrogen and Oxygen is already pretty efficient and getting better by the day. It can use the entire spectrum of light (unlike solar cells which typically only use the IR and red, except for the super expensive multi-band-gap kind used on satellites), and sample implementations up until now have not involved super exotic materials or precious metals like electrolysis does.
According to Nathan Lewis, professor of chemistry at Caltech (and big naysayer back in the days of Cold Fusion, much to the disappointment of us undergrads), it would only take 57,600 square miles of a thin membrane version of this converter to supply the country with all the hydrogen power it needed for transportation and building use. Of course that's more than a couple rooftops.
To give you a sense of scale, Kramer Junction which operates five 30MW power stations and is the biggest solar farm in the world, covers approximately two square miles.