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(Original post) Not the website owner, but isn't an (Android) phone just a small Linux computer? There are a bunch of web server apps (https://play.google.com/store/search?q=web%20server&c=apps) or you can root it and run whatever software you want on it?

In my mind it's not so different than running a web server off a Raspberry Pi or similar.

GoalZero, Jackery, etc. also make them, and it's easy to find no name ones on Amazon or AliExpress.

It's best if you can run a long cable from the panel to the phone and keep the phone under shade (or maybe just under the panel). Many phones will overheat if you keep them in the sun all day.