COLUMBUS, Ohio (AP) — Five years after the worst blackout in North American history, the US's largest power providers say the problems that turned out the lights on 50 million people have largely been resolved, but they fear that larger, systemic issues could soon lead to even bigger and more damaging outages.

Excess capacity in the system is shrinking and construction, as well as plans for new plants, has slowed as costs to build and operate them have soared.

At the same time, it is estimated that electricity use will increase 29 percent between 2006 to 2030 — much of it driven by residential growth, according to a government report issued in June.

"I'm really not a Chicken Little player, but I worry that no one seems to be focusing in on this," said Michael Morris, chairman, president and chief executive of American Electric Power, which runs the nation's largest electricity transmission system.

Morris said massive outages this year in South Africa, which forced gold, diamond and platinum mines to stop production for five days, should serve as a warning to the US.

Industry experts back Morris, and say there is even more resistance to building new plants due to the debate over climate change and opposition to new transmission lines.

"The level of excess capacity has shrunk down in the last few years to a level barely within the planning toleration of the industry," said Marc Chupka, a principal with the Brattle Group, an energy consultant.

The blackout five years ago yesterday shut off power to vast swathes of the Northeast and Midwest for as much as four days. Rolling blackouts continued in Ontario, Canada, for a week.

Hundreds of thousands of people lost access to tap water for days in Ohio and the mayor of Cleveland accused shop owners of gouging people in need of drinking water.

Millions of New Yorkers, with subways shut down and office towers darkened, left the city on foot. A US-Canadian government task force largely blamed Akron, Ohio-based FirstEnergy Corp. for allowing a local power failure near Cleveland to cascade to the East Coast and up into Canada.

Industry experts say changes have been made to protect against a similar outage that caused as much as $10 billion in damages to the US economy.

But Morris fears that in 10 to 20 years there may be greater blackouts as demand surpasses supply.

"Just think of the economic hardship that would render," he said.

Rick Sergel, president of the North American Electric Reliability Corp. of Princeton, N.J., the agency that oversees the nation's power grid, agrees with Morris.