The company uses reading devices from a number of vendors, and the problem doesn't appear to be isolated to a particular batch of equipment, Zagrzebski said.
Only those customers who were undercharged or overcharged will be notified, Zagrzebski said. Where a discrepancy is found, WPSC workers will check and test the meter. The company then will mail an additional bill or refund on the account.
"We understand that people are disappointed that they were back-billed, but they need to keep in mind that they used the power in the first place," said Zagrzebski, who stressed the Wisconsin Public Service Commission mandates the utility charge customers the full amount for their use.
Rhody Mallick of Kristin's At Middletown Food & Spirits in Stevens Point received a back-billing from WPSC. He was told from September 2005 to January 2007 he was undercharged about $700. He set up an 18-month payment plan to square the bill.
"I don't understand how they can say I used this much gas. How do they prove that? How do they know?" Mallick asked. "Business is tough enough the way it is, and then you get hit with an extra $700. It takes a bite out of you."
Mallick said he felt he had no choice but to pay the $700.
State Rep. Louis Molepske Jr., D-Stevens Point, said he can understand customers' frustration.
"The Public Service Commission has room to be more customer-friendly," he said. "Most people aren't sure where to turn when they have a problem. They just want to know their rates are fair and justified."
In January, the Public Service Commission approved a 6.6 percent rate increase for electricity and a 3.8 percent increase for gas for WPSC to expire at the end of the year. The Green Bay-based company, which serves northeastern and central Wisconsin and an adjacent portion of Upper Michigan, reported a $129 million profit last year.