The Energy Secretary has welcomed a £12 million redress package, after regulator Ofgem found energy firm EO.N had broken sales rules.
Ed Davey said: "It's right that if energy companies aren't fair to their customers, then they're penalised - and their customers benefit.
"That's why we introduced legislation to ensure Ofgem can take tough action in these cases, including making the company pay compensation to the people affected."
E.ON has said it will compensate customers it may have mis-sold to. Find out how to make a claim.
E.ON boss Tony Cocker has apologised after the energy firm was found to be misleading customers, according to an investigation by Ofgem.