A common bit of wisdom about solar electric systems is you can start small and add panels later. That's true and it isn't.
I built my panel rack to hold up to 12 panels. Started out with 8 with the idea of adding more later. Even predrilled the holes for the additional panels. By the time I was in a position to buy more panels, that model was no longer made.
The original panels are wired up in pairs for a nominal working voltage of 24 volts. I got a good deal on a large single panel rated at 24 volts. Should have been fine, so I thought. The new panel really didn't work that well with the old panels. It tested at a slightly lower voltage than the old ones.
The original system ran at 24 volts at the panels to charge a batter bank running at 24 volts. I really wanted to run the battery bank at 12 volts, but there was a problem. My panels are far enough from the battery bank that there would be too much power loss. The higher the voltage, the less transmission loss. Back in the day, if you ran panels at a certain voltage, everything ran at that voltage -panels, charge controller, batteries, and inverter.
Fortunately, technology marches forward. The solution to the panel problem was to buy an Outback charge controller. It allowed me to run the old panels and the new panel in series at a nominal 48 volts. The output from the charge controller is programmable. It was fairly easy to set it to put out 24 volts to charge the batteries. In 20 years solar electric systems have come a long way. My old charge controller worked, but not nearly as efficient as the Outback.
Why didn't I drop it all the way down to 12 volts like I originally wanted? The Outback could just as easily been set to 12 instead of 24 volts.
The problem is my old Trace inverter, a model 2524. It works on 24 volts DC and puts out 120 volts AC to run the house. I've had it for twenty years, but it's given zero problems. Once the inverter needs to be replaced, then I'll drop down to a 12 volt battery bank.
Always wanted to run a few 12 volt electronics in the house. It is possible to tap 12 volts off a 24 volt system. Problem is that the batteries would discharge unevenly. I did it for a while and kept rotating which batteries I'd tapped. It wasn't an ideal situation, as I'd either have to test each battery to see how it was discharging, or just out and out guess. There's the option of a DC to DC converter that would drop the voltage from 24 to 12. Problem with that is there's a significant power loss from the device itself.
Instead of buying a fairly expensive Outback charge controller, a cheap charge controller for just the single new panel would have worked. It would then be two separate charging systems running in parallel to a single battery bank. That option seemed like a bit of a kludge. Much better to upgrade to a better charge controller and run the panels at a higher voltage. My original charge controller was almost 20 years old, after all.
So there you have it, it is possible to expand a solar electric system later. My advice is not to wait almost 20 years like I did. The expansion can be a bit more involved than bolting on a few panels.