New England will need to add power plants capable of generating 4,300 megawatts, and $3.4 billion of additional transmission investment, by 2015 to avoid blackouts, the region’s grid operator says.

The area will need 170 megawatts of new power before the summer of 2009 to assure adequate supplies, according to ISO New England Inc., the power grid and wholesale market operator that serves the region’s 14 million people.

Last year, the six-state region added a net 11 megawatts of generation, enough for about 8,800 average U.S. homes, according to Federal Energy Regulatory Commission Chairman Joseph Kelliher.

“Without the timely addition of new resources, the region will fail to meet established reliability criteria, increasing the possibility of needing to disconnect customers during periods of peak demand,” according to the ISO’s 2006 Regional System Plan, which was released yesterday.

New England currently has proposals to build 37 projects capable of generating 3,871 megawatts, the report states.

ISO New England is relying on what it calls “tie-line benefits,” the receipt of electricity over high-voltage transmission lines from neighboring areas during periods of emergency need. The operator said with 2,000 megawatts of tie-line benefits it will need just 170 megawatts to meet “resource adequacy criteria” by 2009.

More generation would be needed sooner if the “assumed” 2,000 megawatts were unavailable.

The report also said the region needs to develop power plants fueled by something other than natural gas, since “the region’s 40 percent dependence on natural gas to generate electricity is a serious reliability risk to New England customers, especially during winter peak-demand periods.”

The report says there are insufficient plans for renewable energy power generators, including wind and solar facilities, to meet state requirements by 2015.

A multistate agreement to limit carbon dioxide emissions from power plants in the region will also require alternative electricity suppliers, ISO New England said.

Four New England states — Connecticut, Maine, New Hampshire and Vermont — have signed on to the Regional Greenhouse Gas Initiative. If Massachusetts and Rhode Island were to join the group, new low- or zero-emission plants would be needed by 2010, the report said.

If a 1,000 megawatt coal or nuclear power plant had been installed in 2005, buyers in the wholesale market would have saved $600 million in power costs, the report said.

New England has 31,000 megawatts of generating capacity and 8,000 miles of power lines.