CPS Energy issued a formal apology Tuesday to customers after months of complaints about the utility's estimated bills practice.

CPS officials said quality issues with contract meter readers led to employees having to manually double check meter readings, and that ultimately created a back-up in the billing process.

Some customers went months without getting a bill, then suddenly received a large bill to cover those months of service.

"We apologize for any of the inconveniences, all the inconveniences we put our customers through," said said Maria Koudouris, CPS Energy Senior VP Customer Service. "We are working with our customers to make sure we get them correct bills, we get them timely bills. For those folks who receive multiple bills at one point, or at the same time, we will work with them to offer them payment arrangements."

CPS added staffing changes were made and CPS is down to estimating about 1.5 percent of meters which is not uncommon for utility companies.

At one point earlier this year, about 22 percent of meters were being estimated.