EDF Energy breached new rules by failing to record customer complaints properly, the regulator has found.
The breach, between October 2008 and March 2009, has now been corrected and the company has made a £200,000 payment to charities which help consumers.
As a result, it has not been fined by the energy regulator Ofgem, following the first investigation under rules introduced in 2008.
The company said it had taken immediate action to rectify the situation.
The regulator brought in the new rules in a bid to improve customer service by energy companies.
It wanted people to be able to call about disputed bills or meter readings, and for them to have to explain the problem only once. These complaints should then be properly recorded by the company.
If any energy company fails to resolve a problem to a customer's satisfaction within eight weeks of the initial complaint, then the customer has the right to take the issue to the independent Energy Ombudsman.
Ofgem found that EDF Energy had failed to record all complaints properly, as demanded by the regulations.
Following an audit, Ofgem launched an investigation. The company then set up workshops for staff and changed its training programme to ensure complaints were recorded properly.
"When Ofgem first identified in January 2009 that the number of complaints we had recorded was lower than expected, we immediately instigated a review and developed a robust action plan to resolve the matter," the company said.
"The actions we took led to an almost immediate and sustained improvement, such that our recorded complaints are now fully consistent with our expectations and with other suppliers."
The company also paid £100,000 to the Money Advice Trust, which assists people with debt problems.
It also gave £100,000 to the Energy Best Deal campaign, run by Citizens Advice, which ensures people are on the correct benefits and the most appropriate energy tariff.
"In light of these actions, Ofgem has decided in this case not to impose a financial penalty on EDF Energy," the regulator said.
The two charities welcomed the payments as directly assisting those in need of help with dealing with fuel bills. Generally, fines go to the Treasury, which can then offer grants to fund schemes.
Ofgem has the power to fine an energy company up to 10% of its global turnover if it is found in breach of regulations.
Audrey Gallacher, from watchdog Consumer Focus, said: "Nothing frustrates customers more than the hoops you are forced to jump through to get a problem acknowledged and dealt with.
"It is vital that energy companies handle complaints effectively and with the attention they deserve."