Xcel Energy is replacing about 23,500 natural gas meter readers in Fargo, West Fargo and Grand Forks because many of them have malfunctioned, a utility spokeswoman said.

Public Service Commission President Susan Wefald said the faulty equipment caused billing mistakes for about 4,400 Xcel natural gas customers in North Dakota last winter.

The problem was detected in late January, when some North Dakota ratepayers noticed their bills did not include any gas use in the middle of winter, David Sederquist, an Xcel regulatory consultant, said in a PSC filing.

Wefald said hundreds of meter reading units, which allow the utility to read its natural gas meters remotely, broke after ice got into them.

In June and July, Xcel began sending affected customers large bills that estimated their natural gas use during the months it was not recorded correctly, Wefald said. The bills prompted a customer uproar.

"We understand that this has been very difficult for the customers in the Red River Valley who are served by Xcel … and the commission is taking this issue very seriously," Wefald said.

Sederquist said most of the 4,400 defective modules were replaced by March.

Bonnie Lund, an Xcel spokeswoman, said the company is installing new components in 23,500 meter-reading modules of the same type, even if the units have been working correctly until now. That work should be finished by year's end, she said Wednesday.

The public service commissioners are questioning whether state rules give Xcel the authority to send estimated bills because of the meter reader malfunctions. In any case, the bills were difficult to comprehend, the commissioners said.

"Customers are suspicious enough of their utilities without getting something so complex that even a Ph.D. in economics can't understand it," Commissioner Kevin Cramer said.