Tampa Bay area electricity customers will see monthly bills increase as much as $9 on average beginning Jan. 1, after state regulators gave approval Monday to final rate adjustments for 2014.

The Public Service Commission unanimously affirmed the rate hikes, which were widely expected based on previous utility agreements and settlements with consumer advocates.

Duke Energy will charge the most of the state's three largest investor-owned utilities with a rate of $125.29 per 1,000 kilowatt hours of use, the amount consumed by the average residential customer. That's up from this year's $116.06.

Tampa Electric customers' rates began rising last week as part of a deal with the state. On Nov. 1, Tampa Electric, a subsidiary of TECO Energy, raised rates by $5.68 and will add another $1.35 on Jan. 1, to go from $102.58 to $109.61 per 1,000 kilowatt hours of use.

"Today's vote was an annual adjustment to reflect the prices of fuel and other factors that are related to the cost of producing electricity," said Cherie Jacobs, a TECO spokeswoman. "Tampa Electric bills remain among Florida's lowest and will continue to be below the national average."

Even so, Tampa Electric and Duke customers will pay about $10 to $25 more than those who receive their power from the state's largest utility, Florida Power & Light, which serves South Florida.

The increases are driven by a variety of factors, including changes in fuel costs, capital improvements and the need to compensate for declining demand.

In Duke's case, part of the increase in rates is related to costs from the broken and shuttered Crystal River nuclear plant.

Customers are on the hook for about $3.2 billion in costs tied to Crystal River and the now canceled Levy County nuclear project. Some of that money has been paid, but the balance will carry forward for years.

As part of settlement agreements with the state, Duke is giving its 1.7 million Florida customers more than $1.5 billion in credits.

Charles Rehwinkel, deputy state public counsel, who represents consumers before the PSC, questioned how Duke was accounting for the credits to customers in its filings. He asked whether Duke would be willing to give a clearer breakdown of the customer credits regarding the nuclear expenditures in the future.

Commissioner Eduardo Balbis also described the current breakdown of credits as "confusing."

Consumer advocates told the PSC they believed all the credits are reflected in Duke's accounting. Still, Thomas Foster, a Duke manager for rate issues, said the utility in the future would provide more information.

"We certainly would be willing to work with folks … to make it easier to follow," Foster said.

Added Balbis: "I hope that whatever you come up with will make it a lot easier."

Ivan Penn can be reached at ipenn@tampabay.com or (727) 892-2332.