At least 7.3 million homes and businesses on the US East Coast were without power on Tuesday after Hurricane Sandy tore down power lines, flooded networks and sparked an explosion at a Consolidated Edison substation on Manhattan's East River.

Almost a quarter of New York City homes were without power 13 hours after Sandy roared ashore with hurricane-strength winds, felling trees and bringing a dangerous tidal surge that has paralyzed the transport system in the United States' biggest city.

Power providers reported outages in every state from North Carolina to the Canadian border and as far inland as Ohio and Indiana. At 9:00 a.m. EDT (1300 GMT), outages were approaching the total from Hurricane Irene last year, according to Reuters calculations based on figures provided by utilities.

Outages caused by Hurricane Irene peaked around 8.4 million. The total outage caused by Sandy is likely higher as it does not include homes that have already been restored or the smallest power providers on the East Coast.

The figures represent homes and businesses, meaning the total number of people affected will be far higher.

"This is the largest storm-related outage in our history," said John Miksad, Con Edison's senior vice president for electric operations.

An explosion at a substation on Manhattan's East River on Monday night contributed to the power cuts, and could complicate efforts to restore power. The vast majority of the island below 39th street - just south of Times Square - was without power.

Over 700,000 homes and businesses were without power in New York City and Westchester county, out of a total of 3 million Con Edison customers.

In New Jersey, power provider Public Service Electric and Gas (PSE&G) said in a Tweet that more than 1.2 million customers were without power, "making it the largest storm in PSE&G history."

The company asked customers to be patient as "unprecedented" flooding threatened to leave homes without power for days.

Customers in the hardest hit areas could be without power for more than a week, utilities have warned.

"We expect that some customers will be without power for as long as seven to 10 days," said the state-owned Long Island Power Authority (LIPA), which has 84 percent of its 1.1 million customers in New York without power.

One forecasting company predicted economic losses could ultimately reach $20 billion nationwide, only half insured.

Connecticut Light and Power's website said 39 percent of its customers were without power.

Back-up generators failed at New York University Hospital in Lower Manhattan on Monday night, forcing patients to be moved elsewhere for care.

Con Edison workers were also trapped for three hours by rising flood waters inside a power substation before being rescued by firefighters using inflatable boats.

Power providers emphasized that customers must stay away from downed power lines. One woman in New York City was killed after stepping into an electrified puddle. – Reuters