Welsh energy supplier SWALEC says its prices for electricity and gas will rise by 9% from 15 October.
In a letter to customers, Chief Executive Ian Marchant says that supply costs "have increased significantly" over the last year, and these pressures have made a price increase "unavoidable."
Increasing prices is one of the hardest decisions any business has to take, particularly in difficult economic conditions, so I am very sorry that we’ve had to announce an increase from 15th October.
Over the last year the costs we face have increased significantly including the charges for delivering electricity and gas to your home, the cost of environmental and social obligations, and what we have paid for the energy we’ll supply to you this winter.
SWALEC says it will be contacting customers before 15 September to give them more information about the rises, and promises there will be no more price increases before the second half of 2013 at the earliest.
More top news
Ched Evan's fiancé Natasha Massey talks for the first time about why she has stood by him for the last five years.
Leanne Wood has said she accepts the referendum result but that it has left Wales with a weak position for negotiation.
It will be a dry start to the day and any fog and low cloud will be slow to clear. Sunny spells will develop into the afternoon.