Welsh consumers pay average annual energy bills of £1089
Suppliers are charging Welsh consumers £19m more for electricity than in England, a consumer watchdog says.
Energywatch said customers pay £14 a year more for their power supply, with research showing households in south Wales are the worst hit in the UK.
They pay 5% more for electricity than consumers in north Wales and 10% more than those in England and Scotland.
Energywatch is calling on the regulator Ofgem to review the way prices are set so consumers are not disadvantaged.
Wendy Davies, Director of Energywatch Wales, also said the government should refer the issue to the Competition Commission for investigation.
"The research confirms what we have suspected for some time, that consumers in Wales are being charged more for their electricity than consumers in Scotland and England and for no good reason," she said.
"£14 a year more on an electricity bill may not sound like a lot but collectively the suppliers are making millions of pounds of profit from Welsh consumers who are struggling to cope with average annual energy bills of £1089. "
Energywatch said its research, which was conducted in April, also found even after 10 years of competition in the market, many suppliers typically charge consumers in their original monopoly area more than those in other regions.
Ms Davies said: "This finding demonstrates very clearly that loyalty simply does not pay.
"Anyone who has remained loyal to their original gas or electricity supplier, having never switched would be wise to look around for a better deal."
Last month, Energywatch Wales said it was concerned that 270,000 households were classed as "fuel poor," up from 130,000 in 2004.
Factors including low income, rising prices, higher supplier costs and prepayment meters all contributed to the size of the bills.
Plaid Cymru AM and spokesperson for social justice, Dai Lloyd has called for the establishment of an independent energy ombudsman for Wales.
"The current charging inconsistency is nonsensical and the public needs a strong voice to fight their corner,” Dr Lloyd said.