“I work in green [technology] for an international development bank, [and] I’m always saying, ‘Look, the costs are down, there are all these incentives, really it makes economic sense for you,'" he says, describing how he convinces others to go solar.

But this year the Washington, D.C., resident decided to take his own advice.

"It’s nice to be able to see that actually happening for me, too, in the United States,” he says.

For years the high cost of photovoltaic (PV) panels, which convert sunlight into electricity, meant that only wealthy Americans could afford them. But that cost has been falling steadily, and panel installations in the U.S. jumped 31 percent in 2011, many of them on middle-income households.

A Group Enterprise

In D.C., 17 neighborhood networks -- so-called solar cooperatives -- have sprung up over the past six years. Mike Barrette, who started one of these co-ops just down the street from the U.S. Capitol, says recent changes in the market have only worked to his advantage.

"We interviewed vendors [and] brought experts in," he says. "We started talking the language, learning the acronyms and all that. I put on a 4.4-kilowatt system in 2010, and since that original decision, the prices of solar have dropped significantly, about 30 or 40 percent."

That drop, combined with federal tax credits, rebates and and utility-cost savings, means that a typical residential solar-power system can pay for itself fairly quickly.

For D.C.resident Sean Carroll, it was these additional factors that finally persuaded him to take the solar plunge.

"For a while people expected it would take five, six, seven years to get a return on investment, [but] now, with these rebates and tax credits, it’s down to about a year or a year and a half," he says. "Once you see that you break even pretty quickly, it makes you say, 'Wow, of course I’m gonna do this'.”

Carroll and other co-op members have also saved money on their PV panels and installation costs by negotiating group discounts.