@descartez you'd need a battery for overnight use too... So I'd suggest 2 portable solar panels (I have a RavPower that can run a tablet all day or charge a battery... Technically both together but not well -hence 2), to keep a battery topped up and run the pi. In theory 5v @ 3.5a should be enough, but go for more area to cover bad-charging days.
@descartez add a bit more power and you could also run a 4g dongle to cover Internet, and make it completely self sufficient.