20€ for 3 solar panels and a powerbank.

https://www.conrad.com/en/p/voltcraft-solar-charger-5w-powerbank-capacity-mah-ah-4000-mah-2127239.html

The powerbank itself isn't that big. But it charges fully in 4 hours and it is really easy to cut them up and extend the cables to put the solar panels wherever you want. And it powers every USB device.

You could also theoretically solder the panels in parallel to charge even faster. But I just connected 2 of the powerbanks with a USB splitter to get 1 USB output.

They also charge the powerbank with the slightest of light. Even just a light bulb is enough for it to start charging, lol.