Energy firms along the Eastern Seaboard scrambled on Monday to resume operations after Hurricane Irene left 5.5 million customers without power.

Utility firms whose transmission lines were battered by the storm over the weekend faced the most daunting workload, with millions of homes and businesses from North Carolina to Maine still cut off. They said the work would take days, and up to weeks in the hardest-hit zones, as they dealt with flooding and debris.

Irene was weaker than some expected, but it will probably take a week to restore power to some areas, said energy analyst Peter Beutel of Cameron Hanover in Connecticut.

The storm shouldn't have a permanent impact on energy infrastructure.

ConocoPhillips' 238,000 barrel per day (bpd) Bayway oil refinery in Linden, New Jersey was due to restart on Monday after closing Saturday, a source familiar with the plant said.

Sunoco shut a crude unit at its Philadelphia refinery after a pump was flooded, sources said. The company was boosting output at another Pennsylvania plant, Marcus Hook.

Other East Coast refineries that throttled back for Irene were resuming normal operations.

One nuclear power reactor at Constellation Energy's Calvert Cliffs facility in Maryland remained shut after being struck by wind-blown debris on Sunday, but the company said the plant was safe. Other plants that reduced operations were preparing to restore normal rates, while Exelon Corp's Oyster Creek New Jersey plant, which supplies up to 600,000 homes, remained offline.

Power Grid Hit Hardest

By Monday, only a small portion of the 6 million customers whose power was cut off by Irene had their electricity restored. The U.S. Department of Energy (DOE) estimated that 5.5 million customers were still affected as of 10:00 a.m. EDT (1400 GMT).

New York, New Jersey, Connecticut and Virginia -- where the outages were greatest in number -- all had more than 600,000 customers without power on Monday.

In Rhode Island around two-thirds of all customers had no power, or nearly 275,000, the DOE said.

In New York State, where 939,000 customers were down on Monday, electric companies had managed to restore electricity to only 4,000 users overnight, the DOE figures showed.

Consolidated Edison, which powers New York City and nearby Westchester County, said nearly 100,000 remained affected in those areas. It expected to restore city customers by late Tuesday and others by Thursday.

Utilities warned that work could be slow-going.

There are still areas we can't get to because of flooding and debris, said Jersey Central spokesman Ron Morano, who estimated that restoring service would take several days. Around a third of the firm's 1.1 million customers in central and northern New Jersey were affected on Monday.

Pipelines Nearly Back to Normal

The New York Harbor, a shipping hub for millions of barrels a day in crude oil and refined products, said it expected to resume normal activities on Monday after Irene's approach forced the port to restrict vessel traffic. The Port of Philadelphia, which serves nearby refineries, also reopened.

The 2.37 million bpd Colonial pipeline system, which ships refined oil products from the Gulf Coast to New York, said on Monday it was nearly ready to resume normal operations. The storm cut power to some oil terminals supplied by the pipeline.

Kinder Morgan's 600,000 bpd Plantation pipeline from North Carolina to Washington D.C. resumed normal operations on Monday after a brief shutdown during the storm.