The Solar Impulse left Moffett Field in Mountain View just after dawn.
It plans to stop in Phoenix, Dallas, Washington and New York. Each flight leg will take 20 to 25 hours, with 10-day stops in each city.
The plane is powered by about 12,000 photovoltaic cells that cover massive wings and charge its batteries, allowing it to fly day and night without jet fuel. But its low weight makes it vulnerable to bad weather.
Its creators say solar planes will never replace fuel-powered commercial flights. The goal is to showcase the potential of solar power.
The plane has already impressed aviation fans in Europe.
COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? email@example.com