Energy Secretary Steven Chu announced a conditional guarantee to cover 80 percent of a $344 million loan for the biggest residential solar rooftop project ever in the United States.

A company called SolarCity plans to install, own and operate up to 160,000 rooftop installations on as many as 124 military housing developments in 33 states over the next five years.

That would equal the current number of residential rooftop installations in the United States. It would provide 371 megawatts of generation capacity.

The Energy Department loan guarantee will significantly lower capital costs for the project, said one of the backers of the project.