An investigation by Ofgem found nearly 250,000 accounts were affected between February 2021 and September 2023 due to a billing system error.
Around 100,000 of these accounts were in credit, with an average balance of £51.
The company failed to issue final bills within the required six-week timeframe, leaving many customers unaware of outstanding credit.
Compensation payments under Ofgem’s Guaranteed Standards of Performance were also not made.
E.ON Next has since rectified its billing system and will pay £14.5m in refunds and compensation, including writing off debts for nearly 150,000 former customers.
Any unclaimed credit will go to the Energy Industry Voluntary Redress Fund to support vulnerable energy users.
Beth Martin, director for consumer protection and competition at Ofgem, said: “Prepayment meter customers are more likely to face financial difficulties, and during a period where households have been facing a significant cost of living crisis, it’s unacceptable that consumers did not receive refunds for credit that was owed to them, or final bills they are entitled to.
“E.ON Next has demonstrated an understanding of the significant impact this issue may have had on its customers, and it’s encouraging that they self-reported the issue and have worked with us to resolve it, and compensate affected customers to put things right.
“The action to write off debt will also offer affected households peace of mind.”
Chris Norbury, Chief Executive Officer of E.ON UK said: “We are deeply sorry to the customers affected by this system issue and have taken the correct steps to put things right.
“As soon as we became aware of the error, we acted swiftly to report it to Ofgem and have since made significant changes to prevent it from happening again. We are committed to putting things right for our customers.
“We have contacted all those affected directly to ensure they receive any outstanding final bills, refunds owed and compensation. This includes ensuring that those who were in credit receive the money they are entitled to and those who were in debit when they left having that money owed removed entirely.”
