Talk about "sticker shock."

"When I saw it, I thought it was mistake," said Sandra Rubio upon receiving a whopping $8,000 energy bill. A few days later, she received a letter from CPS Energy telling her a clerical error had caused her to be underbilled for nearly five years.

The utility, owned by the city of San Antonio, Texas, apologized but said the mother of three would have to pay the bill in its entirety.

"It's gonna be real hard," said Rubio in an interview with CNN Radio. "My boyfriend is the only one in the home who works. There are days when it's slow and he doesn't work at all. I have to pay it back no matter what and I feel cheated."

The problem began when a new meter was installed at the Rubio home in 2005, according to Theresa Brown Cortez, a spokeswoman for CPS Energy. An office employee incorrectly entered a number, causing a multiplier to register as "0" instead of "1," she said.

Rubio was not billed for any electric service even though the meter itself was properly tracking the usage.

CPS Energy supplies both natural gas and electricity to the home, Cortez said, and the utilities are billed together. A bar graph on the bill shows how much of the amount owed is for gas and how much is for electricity.

Rubio was billed for gas consumption, as well as base charges for having gas and electric service, she said. However, she wasn't billed at all for electric consumption.

"The customer on their bill could have seen electric service was given to their home, however they had no consumption," Cortez said.

But Rubio said she saw nothing unusual on her statements stuck out to her.

"We don't know how to calculate those numbers and what those numbers mean," she said. "You trust that whatever they send you, whatever that amount is, that's your bill."

Rubio said CPS told her that for the next seven years she will have to pay $98 a month on top of her regular monthly bill to pay off the debt, She worries that unforeseen circumstances could make this payment plan very difficult on her.

"It's already hard trying to pay three or four hundred dollars a month. What if one month I can't pay my full amount? I'm worried they're going to turn my lights off," she said.

Rubio plans on seeking some form of assistance to help her pay her debt, but she isn't optimistic.

"I know that anywhere I go they're going to look at it and say it's too much," she said. "I don't even know what I'm going to do. I'm just going to try and see what happens."

"We regret that we have human error in this situation," said Cortez. "We're going to do everything we can to work with this family to make this as manageable as possible to get this bill paid."

Company policy calls for the utility to bring in all money owed for services rendered, Cortez said. The company's sales revenues support the city budget, she said.