LOS ANGELES (Reuters) - The United States installed 832 megawatts of photovoltaic solar panels in the second quarter, a 15 percent increase from the prior period as booming business for utility-scale systems helped offset weaker demand for residential and commercial projects, according to an industry report.

Compared with the same period a year ago, the market grew a modest 1.5 percent.

According to a report released Thursday by research firm GTM Research and the Solar Energy Industries Association trade group, the U.S. residential solar market was flat from the prior quarter as installations fell 8 percent and 18 percent, respectively, in key solar markets Arizona and Hawaii.

Solar developers in Hawaii reported that changes in building permit fees and the state solar tax credit have made that market more difficult this year. In California, residential systems grew 4 percent from the first quarter.

Commercial installations fell 11 percent due to regulatory uncertainty and to reductions in incentives in markets like Massachusetts and Arizona.

The utility-scale solar market soared 42 percent, but the report noted that utilities in California and other key states have slowed down their procurement of power from new solar projects.

The average installed price of solar systems fell 9.3 percent to $3.05 per watt from $3.36 per watt in the first quarter. A year earlier, the average price was $3.43 a watt. Prices vary greatly state to state and project to project, however, GTM said. Residential system prices, for instance, ranged from $3 to $8 a watt.

(Reporting by Nichola Groom; Editing by Bob Burgdorfer)