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
UK Weather: Rain in the northwest , could be heavy
Mary Lou McDonald called for action as she clashed with Taoiseach Leo Varadkar.
Ireland’s first openly gay premier said his election would have been ‘unimaginable’ decades previously.