I'm only half kidding. I'm a bit of a prepper and I have lots of powerbanks and devices that charge from USB but besides idling my truck I really had no other way to charge any of them in case of a long-term power outage which seemed a bit of an oversight on my part.
Not like this solves the issue. 30 watts (under ideal conditions) isn't much but it's a start.
There is some inefficiency in charging a battery but not that much. Main losses would be that the solar panel isn't actually outputting 30w as you won't be getting ideal conditions all day.
But it doesn't necessarily matter either. 65w to charge a battery but you only need it once a week for an hour can easily be managed by 5w all day for a week.
Though planning for an apocalypse is kinda pointless anyway. Preparing for a disaster is more reasonable, such as a storm knocking out power for a while.