What are Solarpunk economics? An economic system is a system for the allocation and distribution of goods and resources -- in other words, how you get stuff. (And also services, which may or may not be 'stuff' depending on how you define 'stuff'.)
The two kinds of system that most people are aware of are capitalism and marxism. Marxism however is a social system, not an economic system -- it says that resources should go to people according to their needs, but Marx never specified how this was supposed to occur, and his followers implemented a command-and-control system that would be more accurately called Leninism than Marxism. And didn't work, so I'm not going to discuss it further.
Capitalism is an economic system, but it has also become a religion. It functions OK as an economic system, but as a religion it functions as an uncontrolled positive-feedback loop -- i.e., it doesn't work. This can be summarized as "All for one, and I'm the one!" (Note that this is a summary of capitalism-the-*religion*, not capitalism-the-economic-system.)
I like to use the metaphor of the physical body for the body politic. As an individual human, you are a cell in this body. The economic system is the bloodstream, as your bloodstream is what moves resources around in your body. If we define a state in which the *purpose* of your body is to move the resources around as quickly as possible, we've just defined hemmorhagic fever -- ebola. The consequences of this are the resources ending up very quickly on the floor instead of in the economic (circulatory) system, and is very messily fatal.
Are there other options? Yes. You might want to check out Sacred Economics, by Charles Eisenstein. Speaking as an implementations guy, I don't know how to implement his system, and by my concepts of implementation, neither does Mr. Eisenstein. This doesn't mean it can't be done, and doesn't mean Mr. Eisenstein can't do it -- but I haven't seen or heard him describing a way to make it happen that I could use as a plan for actually making it happen.
I look forward to much vigorous discussion on what we want Solarpunk economics to be, and how to make that come about, and how to make sure it *works* once it does come about. For the moment, I see us as continuing to use capitalism as an economic system, but trying to divorce it from its role as a belief system and create better belief systems.