Monday was a good day for Mosaic, a startup that created a crowdfunding platform for solar projects. After two hard years of setting up the business, which involved getting regulatory approvals, the company opened projects for investment by the public in New York and California.

Within eight hours 335 people invested more than $225,000 in solar projects on Mosaic's web platform at JoinMosaic.com. They invested on average $670. Two projects, including a 55 kilowatt installation for an affordable housing complex in California,...