I believe mailbox.org is all renewable, and I’m pretty sure it’s solar.
But you need a massive battery bank to run stuff, batteries have a limited lifespan (especially the crap used in a UPS).
It’s not cheap, you generally want to overbuild everything, and there are ongoing costs (hardware failures, batteries, etc).
But it can be done. Just have to do the math for your max power draw, then how much uptime you need determines the size of your battery bank and number of panels (which is influenced by how much sun you get/how consistent it is). You need enough panels to run your system and charge batteries, given the limitations of sun availability.
I like your thoughts on runtime and recharge time.
That four hour limit really outs things into perspective for someone just starting out. Most people don’t understand the constraints at first.