Growth in U.S. solar energy installations remains strong — very strong.

That’s according to GTM Research and the Solar Energy Industries Association (SEIA), which together recently released their quarterly report showing that two-thirds more solar photovoltaic (PV) capacity was added in the first quarter of this year than during Q1 of 2010. At the start of April, the cumulative size of all grid-tied solar installations stood at 2.85 gigawatts — enough oomph to power about 600,000 U.S. homes.

Nearly half of all solar-panel systems were installed in sunny California, with about one-sixth in New Jersey, the country’s second-biggest solar market. The rest were installed in Arizona, Pennsylvania, Maryland, Colorado, New York and other states.

Beyond the growing number of solar installations, the report focused also on domestic solar panel manufacturers, finding that U.S. factories churned out 348 megawatts of solar panels, up a third from a year ago.

While America is making more solar panels and related equipment now than in years past, our share of the global market has shrunk: 15 years ago the U.S. made about 40 percent of the world’s solar panels; by 2008, that figure stood at just over five percent.

Still, it seems there’s a lot to like about the U.S. solar power market.

“On the whole, the U.S. is currently the PV industry’s most attractive and stable growth market,” said Shayle Kann, Managing Director of Solar at GTM Research. “This is reflected in our report’s quarterly market data and in the comments from global suppliers, distributors, and developers, all of whom see the U.S. positioned to nearly double its global market share in 2011 and support a greater diversity of installation types than has been previously seen in any leading demand center.”

See the full GTM Research and SEIA report: U.S. Solar Energy Industry Continues Record-Setting Growth in 2011