I would assume they plan to finance the large installations and then pay off the financing by re-selling the projects to retail investors in increments mapping to approximately 1 panel.
In theory they'd only need to keep enough working capital to fund installations until PTO (power turn on) at which point they could "sell" them to retail investors and turn over the capital for the next install.
In practice I'd be worried about what happens if (or when) the company goes out of business. Does my initial investment disappear and someone else now owns the solar installation?
This feels like an offer to become an investor in a risky solar install without any of the actual rights that come from being an investor in a solar install. If the company fails for whatever reason, your solar install goes to the company's investors and you get nothing.
A much better structure would be to operate as a solar holding and operations company and allow people to invest in the company (eventually publicly traded). This lets the investors keep actual ownership of the company and panels, not just some notional ownership on their platform that disappears the moment anything changes.
The way it's proposed here, they're breaking the traditional investment ownership chain and replacing it with some IOUs on the platform. What you really want is to own shares of the company that owns the solar installs, otherwise you get nothing and have no claim as a creditor when the business stops working.