This website made me reconsider if 99.9999% SLA and always online is really that important for lots of websites. I used to self-host my own server in my own apartment, but gave up on that after my internet provider had their first major outage that lasted almost a week. Also I didn’t want to worry about the fire hazard, or the server going down when I was traveling etc.
But a solar powered server seems interesting, if the batteries could be made fire safe.
Use a gell cell lead acid battery (the kind that go in regular UPSs). No fire hazzard, ans bonus, they can survive freezing temperatures just fine if outdoors. Downside is replacing a cheap battery every 3 years.