Many of Conwy and Denbighshire's residents will not be eligible for the largest energy price cuts recently announced by the government, according to Plaid Cymru.
The Westminster government has brokered a deal with the "big six" energy companies to save as much as £50 per household on energy bills.
The move aimed to reduce the burden on those who struggle to pay for gas and electricity during the winter months.
However Plaid Cymru says almost a quarter of householders in North Wales will miss out on the full discount because the most substantial cuts only apply to dual fuel customers, where mains gas is supplied.
Thousands of people in Conwy and Denbighshire rely on "off-grid" sources such as oil and bottled gas.
Plaid Cymru’s Ruthin-based shadow energy spokesperson Llyr Gruffydd AM has written to the UK Secretary of State for Energy and Climate Change highlighting the issue and asking how the government proposes to ensure that people who receive "off-mains" energy supplies do not lose out.
Mr Gruffydd, Plaid Cymru AM for North Wales, said: "It's clear that the Con-Dem government at Westminster has not thought of areas such as Denbighshire and Conwy when arriving at its policies on energy. Rural areas in particular are far more likely to be off-grid and also have households in fuel poverty.
"It was with great fanfare that they hailed the agreements with the 'big-6' energy companies as a good deal for consumers, but it's far from clear how people who aren't on the gas main will get the full benefit.
"In large parts of Wales, people face a choice between getting all their energy from electricity or using oil, bottled gas or other off-grid fuels.
"This means that people aren't eligible for the cheapest tariff.
"Under these 'dual-fuel' deals, people who are near a gas main can get a discount on their energy bills. It's often these prices which are quoted by Government Ministers in their calculations.
"Plaid Cymru welcomes measures to reduce consumer energy bills, although the real reason for rising prices isn't green taxes but energy company profiteering that needs to be tackled. The average profit that energy firms make from each household has tripled in just three years—from £30 in 2011 up to £53 per household last year and up to a huge £105 this year.
"That is why the Party of Wales has proposed a publicly-owned energy company, Energy Wales, with a not-for-profit model."