A “somewhat frustrated” Mayor Thomas Menino said Thursday that he wants NStar to pay up for the Back Bay blackout, which for some power customers lasted nearly 48 hours.

“The shareholders should pay for this, not the ratepayers,” Menino said while standing in Copley Square. “It’s a much bigger problem than we thought it would be.”

Menino said he was concerned for the small businesses that lost products and the employees of those businesses who were losing out on pay.

He said he wants NStar to help defray the city’s overtime costs as well as the losses suffered by the businesses.

However, Menino might not get that help.

A spokesman for NStar said reimbursement is not likely.

“We don’t provide reimbursement for lost business due to power outages,” said Michael Durand, an NStar spokesman. “We recommend customers contact their insurance carriers to determine if their policies cover them for these losses.”

Menino was not the only official calling for financial assistance for the business community.

Sen. John Kerry on Thursday wrote a letter to the U.S. Small Business Administration and asked that it provide economic relief.

More than 20,000 customers lost power after a 115,000-volt transformer caught fire Tuesday night. Since then, scores of NStar crews have worked to restore power to the Back Bay neighborhood. Nearly all of the power was restored Thursday afternoon with the exception of the Prudential Center.

Making the restoration effort more difficult Thursday was the explosion of three manholes not far from the site of Tuesday’s fire.

NStar officials said the cause of the fire was a failure in a connector that linked the underground high-voltage transmission system to the substation, according to the Globe.

Menino said that once NStar finishes an analysis of what happened, he will appoint a panel to examine the report and incident.

Temporary power is spotty

About 50 generators have supplied much of the electricity that was returned to Back Bay residents since Tuesday’s blackout.

The temporary power supply has caused some problems for police, according to Police Commissioner Ed Davis.

He said that power at some intersections was “spotty,” resulting in traffic signals powering down and requiring officers to step in and direct motorists.