HOUSTON, Sept 13 (Reuters) - Restoring electricity to homes and businesses in Southeast Texas following Hurricane Ike may take as long as a month in heavily damaged areas, utility officials warned as the storm weakened and moved inland.

The U.S. Department of Energy estimated that 2.6 million customers in Texas and southern Louisiana lost power overnight as Ike came ashore at Galveston Island, 50 miles from Houston.

Even before utilities could get helicopters in the air to survey the damage to transmission lines, officials said damage exceeded that of recent hurricanes.

Immediately after the storm, utilities reported nearly all customers had lost power as Ike's far-reaching footprint and wind force knocked out more facilities than Hurricane Alicia in 1983 and Hurricane Rita in 2005.

Houston-based CenterPoint Energy <CNP.N> reported most of its 2.2 million customers lacked power, with the power on only in downtown Houston, the Houston Medical Center and InterContinental Airport -- all areas served by underground power lines.

CenterPoint spokesman Floyd LeBlanc warned that power may not be fully restored for weeks. More than 7,000 tree trimmers and line workers will arrive on Sunday to begin restoring power in Houston.

"At the onset, (the storm) looked a lot like Alicia in intensity and the path it took, but it was so much bigger and stayed over our service territory for a long time," said LeBlanc. "We anticipated losing 1 million customers; we did not anticipate losing 2 million customers."

LeBlanc said 25 to 30 percent of the utility's transmission lines were knocked out of service with the most outages seen in on the eastern and southern areas of the city, an area including the Houston Ship Channel, which is lined with refineries and petrochemical plants.

"We have a big restoration effort ahead of us," LeBlanc said.

A quarter of the nation's fuel production capacity in Southeast Texas was shut ahead of Ike and power restoration will be key to the industry's return.

Entergy Texas, a unit of Entergy Corp <ETR.N,> said 391,000 customers, or 99 percent, were in the dark on Saturday.

Texas-New Mexico Power Co, a unit of PNM Resources <PNM.N>, which serves a small area south of Houston including Texas City refiners, said it lost all 115,000 customers.

Entergy Texas supplies power to homes and refineries in Beaumont and Port Arthur.

"Our system has extensive damage," said Entergy Texas spokesman David Caplan. "We're trying to set clear expectations" that repairs will take weeks.

Other Entergy units in Louisiana, still restoring power after Hurricane Gustav struck that state, saw more power outages from Ike. Entergy's Louisiana utilities blamed 70,000 outages on Ike along with 60,000 remaining Gustav outages.

Entergy's 2,000-megawatt Sabine Power Station, shut in advance of Ike's storm surge, was flooded by four feet of water that will require weeks of repair, Caplan said.

The 2,560-MW South Texas Project nuclear power plant located 60 miles southwest of Ike's landfall continued to run at full power through the storm, operators said. (Reporting by Eileen O'Grady; editing by Mary Milliken and Todd Eastham)

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