Residents and businesses in the 13-state region covered by PJM Interconnect have now installed more than a gigawatt of solar power, enough to power between 800,000 and 1 million homes, and more than doubled solar capacity last year, the grid operator said.

The milestone, announced in mid-May, continues the trend of solar growth in the northeastern and mid-Atlantic territory in the last two years, and reflects a range of incentives offered by states that are striving to reach renewable-energy goals.

“Throughout PJM we are seeing tremendous growth, which aids in meeting the renewable goals of states in our region,” said Andrew Ott, PJM’s senior vice president of markets.

Incentives are provided in the form of Solar Renewable Energy Certificates awarded to businesses and residents who can generate at least one megawatt hour from solar energy. The certificates can be traded or sold to another entity to help meet its state-mandated renewable-energy requirements.

The strong growth in PJM’s solar industry last year was largely driven by state renewable portfolio standards but also by declining system costs and by federal tax credits, which expired at the end of 2011, said Paula DuPont-Kidd, a spokeswoman for PJM.

Another 64,000 mw of capacity is planned for the PJM territory – which also includes the District of Columbia – of which some 38,000 MW is from renewable sources. Among the renewable projects, the vast majority, some 34,000 MW, will be from wind power.

New Jersey and California Blaze Ahead

The latest gain in solar – 139 percent in 2011 – is smaller than the 183 percent increase in the PJM region in 2010.

A good portion of PJM’s solar growth was seen in New Jersey, whose photovoltaic installations totaled 313 MW in 2011, almost three times the rate in 2010, and which was second only to California in total installations, according to the Solar Energy Industries Association.

New Jersey’s burgeoning solar industry helped give PJM the second-largest number of solar installations of any regional transmission organization, after California ISO. PJM’s total solar capacity is more than five times that of the New England ISO where an estimated 129 MW is now operational. Another PJM state, Pennsylvania, installed the sixth-largest amount of PV last year.

New Jersey and California together built more than half of last year’s non-residential solar, which more than doubled nationally over 2010, according to the SEIA’s 2011 report.

Capacity Rapidly Increasing, but From a Small Base

Nationally, there were 1,855 MW of solar built in 2011, more than doubling the level of 2010, spurred by a drop of about 20 percent in system prices as a result of lower component costs, more efficient installation, and a shift toward larger systems, the SEIA said in its annual report.

To access a Federal Energy Regulatory Commission Order regarding PJM and demand response, click here, and additional Breaking Energy coverage of PJM’s dealings with state and federal regulators can be read here.

Within PJM territory, almost three-quarters of new solar projects were less than 10 kW while just 3 percent were classified as large, with a capacity of more than 250 kW. But the large projects generated around 66 percent of the total, which came to 1004.8 MW, or just over a gigawatt.

Despite the strong solar growth, solar energy accounted for only 4.3 percent of energy produced in PJM’s area through the first quarter of 2012.