By RYAN TRACY

ReutersU.S. electricity regulators are studying the impact of historic sunstorms. Pictured, a NASA handout image of the sun.

With a peak in the cycle of solar flares approaching, U.S. electricity regulators are weighing their options for protecting the nation’s grid from the sun’s eruptions—including new equipment standards and retrofits—while keeping a lid on the cost.

They are studying the impact of historic sunstorms as far back as 1859 to see if the system needs an upgrade, and encountering a clash of views on how serious the threat is and what should be done about it.

Among the events they are examining is the Canadian power outage of 1989. On March 13 of that year, five major electricity-transmission lines in Quebec went on the fritz. Less than two minutes later, much of the province was in the dark. The cause: A storm of charged particles from the sun had showered Earth, damaging electrical gear as far away as New Jersey and bringing displays of the aurora borealis, or northern lights, as far south as Texas and Florida.

The sun is expected to hit a peak eruption period in 2013, and while superstorms don’t always occur in peak periods, some warn of a disaster. John Kappenman, a consultant and former power engineer who has spent decades researching the storms, says the modern power grid isn’t hardened for the worst nature has to offer. He says an extreme storm could cause blackouts lasting weeks or months, leaving major cities temporarily uninhabitable and taking a massive economic toll.

“This is arguably the largest natural-disaster scenario that the nation could face,” said Mr. Kappenman.

Mr. Kappenman has consulted for companies that make equipment to harden the grid.

Others are more cautious in their predictions. “We need to carry out more detailed and more rigorous analysis before we know for sure,” said Antti Pulkkinen, a physicist with the National Aeronautics and Space Administration, who is using supercomputers to build models of potential future solar storms based on data that have accumulated for decades.

Most in the industry say that they don’t think the consequences would be so severe but that a lesser event is conceivable and worth preparing for.

In February, North American Electric Reliability Corp., a government-chartered entity that enforces national standards for the grid, said the likeliest consequence of a strong geomagnetic storm would be blackouts in the affected areas. The storms tend to have a greater impact in northern latitudes, in part because of the nature of Earth’s magnetic field. The report said most transformers would stay online, so a blackout would likely last only hours or days.

Officials of American Electric Power Co., AEP -0.45%the largest operator of transmission lines in the U.S., and Exelon Corp., EXC -0.36%one of the nation’s largest power generators, have told regulators they are collecting data on what happens during solar storms to assess weak points. “We tend to know what is vulnerable, and we are acting on it,” said Michael Heyeck, AEP’s senior vice president for transmission.

In a solar storm, charged particles flare from the sun and hurtle into space. When they collide with Earth, the electricity-transmission system acts like a jumbo antenna, picking up currents created when the particles interact with the planet’s magnetic field. Those currents can cause wild voltage fluctuations, overheating and permanent damage to transformers, which zip electricity around the grid. The transformers weigh hundreds of tons each and aren’t easily repaired or replaced.

Regulators have known about the threat of sunstorms for years but have only recently begun to coordinate disparate efforts to study the problem and formulate a response. Sunstorms can also force airlines to reroute flights and can disrupt the operation of commercial satellites and interfere with or damage their power and navigation systems. Military and spy satellites typically are less vulnerable.

The 1989 Quebec storm didn’t cause widespread transformer damage and the outage ended after nine hours. But the two biggest solar storms in recorded history took place in 1859 and 1921, before the development of the modern electricity grid.

Over several days in August and September 1859, observations of the northern lights were reported as far south as Panama and Cuba, according to historical accounts cited in a 2008 report from the National Academy of Sciences. One article from the time in the Rocky Mountain News said a group of campers in the mountains saw a light so bright that “some of the party insisted it was daylight and began the preparation of breakfast,” even though it was just after midnight.

The 1921 storm, which took place in May, produced similarly large displays and caused widespread problems in the national telegraph system, disrupting service from the Atlantic coast to the Mississippi River. By Mr. Kappenman’s reckoning, those events were as much as 10 times as powerful as the 1989 storm.

The Federal Energy Regulatory Commission, which oversees the grid, has begun to look into possible new rules. Chairman Jon Wellinghoff said the four-member commission might require upgrades if it found “the threat was high and the cost was low.” Regulators could require the industry to install blocking devices on transformers, for example, or raise the construction standards for high-voltage gear. Or they might take less intrusive action, like ordering more monitoring devices and additional threat assessment. An April 30 conference organized by the commission saw vigorous debate on how quickly the grid needs upgrading.

“We already know that the danger to society is great enough to warrant taking immediate action,” said Peter Vincent Pry, executive director of the Task Force on National and Homeland Security, a group that members of Congress designated to track electrical-grid risks and that supports Mr. Kappenman’s conclusions. By Mr. Pry’s math, it would cost about $200 million to install blocking devices on existing transformers that serve the 100 largest U.S. cities.

Others disagree. Frank Koza of PJM Interconnection, which coordinates electricity transmission in 13 states, said the cost could rise to hundreds of millions of dollars if transformers had to be replaced. But at the moment, he said, “no one can provide sufficient evidence that an immediate large-scale investment by the assets owner and government would adequately address the risk.”

Mr. Heyeck of American Electric Power said customers will want that evidence before seeing their electricity bills rise.

“We’ve got to recognize that someone out there is paying for all this,” he said.

Write to Ryan Tracy at ryan.tracy@wsj.com