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.
Just light a small fire and keep it pretty small and it can burn out once you are done.
I use a Kelly kettle stove a lot, tiny fire but still a few thousand watts of thermal energy. A few sticks at a time as fuel.