(Bloomberg News) Record tides from Hurricane Sandy combined with hours of high wind and rain to deal an unprecedented blow to the U.S. Northeast's power grid, causing the biggest blackout since 2003.

The storm knocked out power to more than 8 million homes and businesses, according to the U.S. Energy Department. Consolidated Edison Inc., New York City's utility, cut electricity in parts of downtown Manhattan and Brooklyn as seawater encroached on power equipment.

Blackouts that may last 10 days in some places closed the stock market and disrupted operations at refineries, pipelines and power plants. Damaged power lines, substations and other infrastructure will contribute to an estimated $20 billion in total storm costs, according to a forecast yesterday from Eqecat Inc., a risk-management company in Oakland, California.

"This is the largest storm-related outage in our history," John Miksad, senior vice president for electric operations at New York-based Con Edison, said in a statement today. A cascading blackout that stemmed from a power line failure in Ohio in August 2003 cut power to an estimated 50 million people.

Record Storm

Sandy, the largest ever Atlantic tropical storm with winds stretching 1,100 miles (1,770 kilometers) from end to end, was forecast to lift seas as much as a record 13 feet.

"The storm surge and the magnitude of the storm is what's going to set this one apart," said Samuel Brothwell, senior utilities analyst with Bloomberg Industries, in a telephone interview.

"This is a really, really difficult blow to the state, but one we will recover from," New Jersey Governor Chris Christie said at a press conference today. The storm made landfall in southern New Jersey yesterday.

At least 20 deaths caused by the storm were reported in New Jersey, New York, Maryland, North Carolina, West Virginia, Pennsylvania and Connecticut, according to the Associated Press. Some of the victims were killed by falling trees; at least one death was attributed to the storm in Canada. The storm was blamed for 69 deaths in the Caribbean, the AP said.

As New York and Connecticut evacuated hundreds of thousands of residents from coastal areas yesterday, United Illuminating Co., which serves customers in Connecticut, and Con Edison shut substations, cutting off power to prevent equipment damage from flooding.

Extended Repairs

Once floodwaters recede workers may need a week to clean and repair the flooded stations before power can be restored, Brothwell said.

About 90 percent of Long Island is without power, New York Governor Andrew Cuomo said. Crews are coming in from as far away as California to help restore power, he said at a press conference today.

Con Edison shut down power to parts of downtown Manhattan, including Wall Street and the nation's financial nerve center, as a 14 foot storm surge in the area, boosted by high tide, sent salt water pouring into its underground power network.

More than 695,000 of its customers in New York were without power after an explosion at the company's 14th Street substation and two substations in Manhattan and one in Brooklyn were shut. It may take as long as a week to restore power to all customers, Con Edison said yesterday.

Even with the precautionary shutdown, salt-water corrosion will complicate restoration, Matthew Cordaro, former chief operating officer for Long Island Lighting Co., said in a phone interview yesterday. "It introduces a whole host of problems," including contamination that has to be cleaned.

Sandbagging Substations

Jersey Central Power & Light, a unit of FirstEnergy Corp., used sandbags to reinforce 29 of its substations to prevent flooding, Ron Morano, a spokesman, said in a telephone interview yesterday. The company brought in 1,400 line workers and 1,200 tree-cutters from Florida, Canada and nearby Midwestern states to restore power after Hurricane Sandy, the utility said in a statement.

Public Service Electric & Gas Co., the utility for about three-quarters of New Jersey's population, said its sandbagging couldn't prepare it for the "wall of water" that came with the storm.

Northeast Utilities' Connecticut Light & Power was better prepared, "particularly in coordination and communication," than it was last year when contending with Hurricane Irene and an early snowfall that cut power to millions along the East Coast, William J. Quinlan, senior vice president for emergency response, said before the storm hit.

Response Criticized

Eastern U.S. utilities were criticized by customers, regulators and state lawmakers for their handling of those weather events. Damage was worsened for the storms, which each left more than 4 million customers without power for a week or longer, because utilities had neglected tree-trimming for years, regulators said.

For this storm, power companies mustered thousands of line workers from across the country, beefed up call-center staffing and piled sandbags to protect electrical substations knocked out by flooding last year. Added spending on trimming tree limbs near power lines during the year also is expected to lessen the number of blackouts.

The Edison Electric Institute, a Washington-based group that represents publicly traded utilities, warned that power failures from Sandy may be extended by high winds and flooding expected to linger for two to three days. Electrical crews won't be able to begin work until winds subside, flood waters recede and workers clear downed trees.