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
Special 'baby boxes' allowing mothers to give away unwanted babies safely have been installed at fire stations in Indiana, USA.
A motorcyclist has died after a chase involving a UK-registered car ended in a police shoot-out and pile-up on a busy motorway near Dunkirk.
Leicester City manager Claudio Ranieri is confident of keeping his star names at the King Power Stadium next season.