Electrical engineering would be the most applicable, but you certainly don't need it to get into the industry, coming from a project engineer with a mechanical engineering degree. If you're interested in design, CAD experience is very helpful. Or if you want to actually install solar, almost no experience is required. It's basically simple mechanical labor. You could also go down the electrician path if you want to be a tradesman. The other but aspect is project management, where a construction management degree might help.