EDF Energy has acknowledged that its customers were caused significant disruption when the firm introduced a new IT system in 2011 and has publicly apologised for this.
Ofgem said that the firm's payment of £3 million to vulnerable customers was a "step in the right direction".
EDF Energy failed to have sufficiently robust processes in place when they introduced a new IT system and this led to the unacceptable handling of complaints.
Their commitment to putting things right and paying £3 million to the Citizens Advice Energy Best Deal Extra scheme and the Plymouth Citizen Advice Bureau's Debt Helpline to benefit vulnerable customers is a step in the right direction to rebuilding consumer trust.
More top news
Thirty facts about Prince Harry to celebrate his coming of age.
Django Unchained actress Danièle Watts was handcuffed by police after she was seen "showing affection" to her partner, NBC News reported.
A US news anchor has told viewers he only has four to six months to live on air, after he was informed that his brain tumour was inoperable.