Here is the eligibility guidance from the Solar Prize Round 5 Hardware Track Official Rules, Section I.6:
“Individuals, teams of individuals, private entities (for-profits and nonprofits), and nonfederal government entities (such as states, counties, tribes, municipalities, and academic institutions) are eligible to compete in the Ready! Contest.”

“Individuals, private entities (for-profits and nonprofits), and nonfederal government entities (such as states, counties, tribes, municipalities, and academic institutions) are subject to the following requirements:
• An individual prize competitor (who is not competing as a member of a group) must be a U.S. citizen or a permanent resident.
o A group of individuals competing as one team may win, provided that the online account holder of the submission is a U.S. citizen or a permanent resident. Individuals competing as part of a team may participate if they are legally authorized to work in the United States.”