A much better question is asking what happens when it is not sunny? Because the scenario is always constructed in such a way that exaggerates the efficiency of battery cars. Of course, a solar powered car would be even more efficient in that scenarios, but we don't talk much about those.

The problem is that we have to store energy, often for very long periods of time. For the grid, this is called grid energy storage, and usually includes a wide variety of options. One of which is hydrogen itself, since it is the best to store energy for very long periods.

So in practice, there's not much difference in efficiency, since every idea requires some kind of compromise somewhere. BEVs will often need hydrogen to back it up. But the main point is that once you make the switch to some kind of EV, the issue of efficiency is mostly moot, since you already well beyond ICE cars in terms of efficiency. The rest of the argument is a distraction, mostly made by people who want to promote one idea specially.