Energy firm Swalec is increasing its prices for household gas and electricity.
Electricity prices will rise by an average of 11% and gas will increase by 18% in September.
The company, owned by Scottish and Southern Energy, has blamed an increase in the cost of wholesale prices and has said a typical annual dual fuel bill will rise from £1,094 to £1,265.
In a statement the company said: “Three things are putting upward pressure on prices. First, the cost of using the energy networks to distribute electricity and gas to customers’ homes, which is determined by Ofgem, is 14% higher than it was a year ago.
“Second, the cost of the mandatory environmental and social schemes that suppliers are required to fund, like the Carbon Emissions Reduction Target (CERT) and the Warm Home Discount, is 11% higher than it was a year ago.
“Third, the wholesale cost of electricity and gas has gone up by around 23% and around 40% respectively since SSE last announced a price increase for household gas.”
The increases will affect about 5.2m household electricity customers and 3.6m household gas customers.
This includes more than one million customers in Wales.