At 11 p.m., power plant operators had restored 21,300 megawatts of the 61,800 megawatts lost, according to the North American Electric Reliability Council, Princeton, N.J.

"A large number of nuclear plants in the affected areas went off line and may take several days to return to service," but operators should be able to bring most fossil-fired generators back online within a few hours, NERC said.

Although III officials were not able to release a specific cost estimate Thursday, they said they believed that the blackout would be relatively short and that losses would be modest.

"There are specific exclusions in most commercial policies for power failures," said III Vice President P.J. Crowley.

When businesses have coverage against business interruptions, the interruption provisions usually kick in only after interruptions of at least 24 hours, Crowley said.

Although many homeowners policies cover losses due to food spoilage resulting from a power failure, III pointed out in discussion of Thursday's outage that the typical spoilage loss falls below the average homeowner's insurance deductible.

One of the most famous U.S. power failure, a two-day blackout that hit New York in 1977, and the riots connected with the outage led to only $30 million in insured losses, III said.

At press time, authorities were still determining the cause of the power failure. In theory, if authorities found that the blackout was due to negligence, affected businesses might try to sue the party or parties alleged to be responsible. Crowley declined to discuss the possible effects of the blackout on liability claims exposure.