the main problem is that the Pi is really power hungry and the sun is unreliable. it always shines but not where you want it to
I had a look at websites that talk about off grid solar power systems which is basically what you are thinking of. one difference is that when you have a small cabin off grid you normally include a backup generator so you can charge your battery in December and January when there is not enough sunlight. or when the weather is cloudy for weeks.
my conclusion is that the Pi requires at least a 60W panel and a 100Ah deep discharge 12V battery. in many locations you need a lot more if you don't want the battery to die within a year.
in a sunny place with few cloudy days you may get away with a smaller panel and battery, but I doubt it.
I would start with a 60 or 72 cell solar panel (250-300W) of the type that is used for large solar installations, because you get more watts for the same dollar amount. and you need all the charge current to get you battery back after a cloudy week. if you let the battery sit almost empty it will quickly die. with this type of low cost panel you must use either a 24V battery (to use the cheap PWM type of charger) or if you have a 12V battery you need a more expensive MPPT charger.