Small-scale solar installations in the United States account for about a third of the overall capacity on the grid, a report from the federal government said.

The U.S. Energy Information Administration estimates total U.S. solar-power output in September, the last full month for which data are available, at 3.5 million megawatt hours. Of that, just more than 30 percent came from small-scale solar installations.

"Generation from roof-top photovoltaic systems has become an increasingly important part of total solar generation in the United States," EIA Administrator Adam Sieminski said in an emailed statement.

A September report from the Solar Energy Industries Association, with support from green energy market adviser GTM Research, found second quarter residential solar capacity grew 70 percent year-on-year to 473 megawatts.

The report finds that 10 states have installed more than 10 MW of solar power during the second quarter, up from the four reported during second quarter 2013. In its latest outlook, EIA said California is the state with the most solar power, accounting for nearly 40 percent of the nation's total capacity.

The White House said the average cost of solar installations has dropped by 50 percent since 2010. Advancing solar capacity, it added, is difficult because about half of U.S. households and business are renters or lack the funds or roof space necessary to install solar power systems.

With small-scale systems advancing, however, EIA said it's now including its contribution to total solar output for the first time in a monthly series on electric power.

Total U.S. solar power generation accounts for about 1 percent of total reported electricity generation in the country.