President Obama on Thursday visited areas recovering from hurricane Sandy, which knocked out power to more than 8 million people. Power is finally coming back on for the last few in New York and New Jersey who have yet to regain it after the lights went out more than two weeks ago.

But lessons learned since darkness descended on central New Jersey in the wake of Sandy's wrecking winds include at least one tiny triumph: Princeton University's leafy campus stayed lit by tapping its own smaller version of the power grid – a "microgrid."

Microgrids were a hot topic among some policymakers even before Sandy hit. Backup generators may fail to start, run out of fuel, or break down. But microgrids like the one at Princeton act as a highly efficient, miniature version of the big power grid, operate 24/7, and tap into reliable natural gas-fired generators or perhaps wind turbines or even solar panels with battery storage.

Spurred by hurricane Irene and a bad snowstorm last October, Connecticut, New York, and Maryland have had teams researching energy options to hedge against widespread grid outages from increasingly violent storms. Microgrids, they found, can supply power to critical shelters, hospitals, and city centers even if the grid is out for days on end. But they cost a lot to set up and legal barriers have slowed development.

Even so, microgrids are getting a hearing nationally as well as along the stricken East Coast. The US power grid received a D-plus grade from the American Council of Civil Engineers in 2009. Power outages averaged 120 minutes per customer last year and were growing, Department of Energy and industry data show.

"This new storm [hurricane Sandy] will undoubtedly accelerate interest in developing microgrids," says Peter Asmus, a senior analyst for Pike Research, a market research firm that tracks microgrid development.

Minutes after trees fell on utility lines two weeks ago, knocking out power to Princeton, the university's energy manager, Ted Borer, began checking the school's big natural gas-fired turbine generator. Serving 12,000 students and faculty, the generator routinely supplies all the school's heat and hot water and half its electricity – the rest usually coming from the local utility.

After a few minutes in the dark, Mr. Borer flipped switches that restored power to much of the campus including one dining hall, the dormitories, and all the critical lab experiments. Some classrooms and administration buildings remained dark. But for nearly two days the campus was on its own power, which allowed some students to begin organizing efforts to help others in the surrounding city, which was still in the dark.

"We designed it so the electrical system for the campus could become its own island in an emergency," Borer says. "It cost more to do that. But I'm sure glad we did."

Princeton wasn't the only institution enjoying microgrid "islanding" capability:

New York University used its microgrid to provide power and heat to a big part of its Manhattan campus while power was out all around it.

South Windsor High School in Connecticut typically uses a big fuel-cell system to convert natural gas to electricity to defray power costs, but it switched over to become grid independent and served as an emergency shelter during Sandy.

The Federal Drug Administration’s White Oak research facility in Maryland, which supplies much of its own power every day, switched to island mode with its own natural gas turbines powering all the facility's buildings for more than two days.

Few true microgrids exist today. Most state laws are geared to limit competition to big utilities. Moreover, microgrids are expensive. The early-stage research costs alone for a four megawatt microgrid – which could power a town center, for example – could be between $800,000 and $2 million, according to Pareto Energy, a Washington-based microgrid company.

Connecticut is among the nation's leaders in microgrid development. Though an attempt to help Stamford become the state's first major microgrid a few years ago failed, the state is taking a fresh look. Last year, the two storms that hit Connecticut each left more than 800,000 customers without power. In response, the state created a $15 million microgrid pilot project, which is expected to select several test localities this year.

"The main thing we saw from last year's storms is the need for more reliability from our electric system, specifically in areas that have critical facilities that need to be running throughout these emergency situations," says Paul Mounds, senior policy analyst and adviser to Connecticut Gov. Dannel Malloy on microgrid development.

Nationally, there's a push, too. The Department of Defense has at least a dozen microgrids already up and running on military bases with another half dozen pilot projects going. The Department of Energy reports a handful of other microgrid sites under development.

Worldwide, microgrids total 3.2 gigawatts (about equal to three nuclear power plants), according to Pike Research. That's up from 2.6 gigawatts of capacity at midyear. North America is the world’s leading market with more than 2 gigawatts of microgrid power planned, proposed, or under development.

Aside from the benefits of "islanding," microgrids also offer greater efficiency. The loss of energy as heat during transmission over long distances means grid power is about 40 percent efficient, while microgrids often exceed 80 percent, researchers say.

"Given the widespread power loss for such a long period of time in New Jersey alone, microgrid systems and energy-storage systems could play an intricate role in preventing power loss," says Amanda Scaccianoce, a spokesman for Princeton Power Systems, a New Jersey company that designs microgrids.

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Ken Olsen could not agree more. As vice president of facilities planning and construction at Ocean County College, a community college in Toms River, N.J., he watched as hurricane Sandy hit the community head on. But he'll be ready next time.

"We just recently installed a new microgrid system and were just a few weeks from turning it on when Sandy hit, so unfortunately we lost power this time," Mr. Olsen says. "But next time we will go into island mode automatically, and that will give us power and shelter we can help provide to the community."