At a minimum you need: a solar panel, a charge controller, a deep cycle battery, a DC-DC buck converter (to reduce battery voltage to 5V), hardware to mount the panel, something to house all the components.
The trick is in sizing the solar components, and that depends on how much sunlight you get and how much reserve power you want. However, even under the best circumstances, I believe it would be challenging to power a Pi 24/7/365 in most places with anything less than a 30W panel and 12V 5AH AGM SLA battery.
The reason is that the panel has to provide enough power to not only keep the Pi running during the day, but also charge up the battery to keep the Pi running at night. The Pi consumes approximately 1.5 watts. For 24 hours of operation, that's 1.5 watts x 24 hours = 36 watt-hours. A 12V 5AH AGM SLA battery can theoretically supply 60 watt-hours of power. But you never want to drain lead acid more than 50%. Also there are always inefficiencies. So you'll only get perhaps 23 watt-hours from the battery. That's enough to get a Pi through the night—but only if the battery was fully charged to begin with. And that's where the panel wattage becomes important.
A 30W panel can theoretically provide 30 watts per hour at "peak sunlight". But realistically you're only going to get perhaps 23 watts from the panel. So you'll need 1.6 hours of peak sunlight (1.6 hours x 23 watts = 36.8 watt-hours) to power a Pi that uses 36 watts a day. You can check online to see how many hours of peak sunlight your area gets. You want an average for the winter.
Seattle, Washington, for example, only gets 1.6 on average during the winter. So the 30W panel would just make it. But that still assumes you'll get at least 1.6 peak hours of sunlight per day. In reality, you may go days with no sunlight whatsoever. That's why you want to upsize your panel + battery for reserve power.
It would therefore not be unusual to power a Pi with a 150W panel and a 75AH battery. Many would say that's overkill. But it's not if you want reserve power for cloudy days.