E.On has paid £7.7 million in redress to Citizens Advice's Energy Best Deal service, after Ofgem confirmed failings in how it dealt with customers after price rises in 2013 and 2014.
The company had overcharged customers when they switched suppliers after the price rise and in some cases had charged exit fees.
Ofgem said the failings were due to system errors, and a lack of oversight and monitoring control. The company has promised to carry out an end-to-end review of its system and provide independent evidence that its improvements are robust.