Vulnerable EDF Energy customers not currently on one of the company's cheapest packages are set to receive a refund.
The move comes as part of EDF's Our Customer Commitments scheme, which will be introduced by the winter and save eligible customers around £40 a year.
But it's not for everyone and to qualify you must be on the guarantee element of Pension Credit, which works out at around 150,000 customers.
If those eligible are not already on the cheapest tariff, they'll be given a refund of the difference between this cost and the amount they are currently paying.
The company will review bills from 2 April and it will then contact eligible customers, who should receive the first rebate before winter 2012.
EDF says its most vulnerable customers are not likely to switch energy packages regularly and are therefore not on the most competitive deals.
[widget:10667] Its chief executive, Vincent de Rivaz, says: "The energy industry has been criticised over prices, service and complex tariffs and our commitment to ensure those identified as most in need automatically get our best prices continues our track record of innovation in helping vulnerable customers."
Consumer groups have applauded the move. Ann Robinson, director of consumer policy for uSwitch.com, says: "This new initiative is very welcome as it means vulnerable and elderly customers, who are the least able to shop around for a better deal themselves, will automatically get the best prices."