Every month the Department of Energy's Energy Information Administration (EIA) publishes data about energy production and use in the U.S. Until now it had not included information about distributed and rooftop solar power. That changed in December as EIA noted the increased role that small-scale solar installations like rooftop solar makes up roughly a third of all solar power in the U.S.

"Generation from rooftop PV systems has become an increasingly important part of total solar generation in the United States. The new monthly reporting on small-scale distributed solar PV systems gives the country a better way to track their contribution to the nation's electricity supply," said EIA Administrator Adam Sieminski.

Previously much of that information was released on a quarterly basis through the Solar Energy Industries Association (SEIA) and GTM Researchs quarterly Solar Market Insight Reports, which relied on information from solar installers. EIAs Electric Power Monthly gets data from numerous sources, including utilities and third-party owners like SolarCity.

Going forward the office will provide monthly estimates based on reporting information. Its a tacit acknowledgment of the increasing amount of small-scale solar power in the U.S. "Small-scale distributed solar photovoltaic systems, such as those found on residential and commercial rooftops, have grown significantly in the United States over the past several years," EIA noted. "Small-scale PV installations are often called ‘behind-the-meter, ‘customer-sited, or ‘distributed generation capacity. Although each distributed PV system is very small—a typical size for residential PV systems is 5 kilowatts (kW) or 0.005 megawatts (MW)—there are hundreds of thousands of these systems across the country that together add up to a substantial amount of capacity.

"EIA estimates total U.S. solar generation was 3.5 million megawatthours in September 2015, with 33 percent coming from small-scale solar PV and 67 percent from utility-scale solar," EIA said. Between the two sectors solar electric generation still just made up about 1 percent of the total reported electricity generation from all utility-scale sources of 351 million megawatthours in September 2015.

The report also provided some state-level information. The majority of distributed solar power is in California, which had almost 40 percent of the nations distributed solar power. The next top nine states account for another 44 percent of rooftop solar. Other leading rooftop solar states include Arizona, Hawaii, Massachusetts, New Jersey and New York. The EIA attributed a combination of factors that make solar power attractive. Among them is the solar resource itself but also state policy and incentive programs and the high electric prices in some markets.