Solar cells to provide the DC voltage for producing the hydrogen (and oxygen as well, to be vented off) stored for night time use to light your garage while working on your Time Machine.....................
Something I haven’t seen addressed here. For hydrogen to be a practical fuel, at least for transport purposes, it has to be highly compressed, or possibly liquefied. (I’m a little vague on the details.)
Unless I’m even more vague than I think, compressing gases this much requires a LOT of energy. Which might pretty dramatically impact the “efficiency” of the system as a whole.
In fact, I’ve seen plans for cars designed to simply run on the energy stored in highly compressed air.