Ofgem said it had been assured by E.ON that all of the consumers affected had been identified and would be contacted and sent a rebate of an average of £14.83.
The amount includes 8% interest on the amount they were incorrectly charged.
More top news
A memorial service has been held at Birmingham Cathedral to remember the victims of the Birmingham pub bombings 40 years ago tonight.
One of the world richest and most influential philanthropists has told ITV News that governments failed to act fast enough over Ebola.
Chewing gum manufacturers must help pay the multimillion-pound bill for cleaning up discarded pieces, councils have said.