E.ON's customer service director, David Bird, said:
We are very sorry to have let down some of our former customers and have made clear that we will refund the money plus interest. Our systems are being updated to ensure this mistake can never happen again.
We've been open in our failure with the energy regulator, Ofgem, and are pleased to have agreed with them how we can put this right and have identified all customers who are due to receive payment from us in January.
More top news
It will be a frosty Christmas morning across the UK but any chance of snow is limited to the Scottish mountains.
Fainting dogs, dancing grooms and an attacker who picked on the wrong girl all featured highly in our most-read stories of the year.
Some dogs have been at the Battersea Dogs and Cats Home for more than a year. Can you make their Christmas and give them a new home?