The system for calculating most domestic fuel bills in the UK is contributing to thousands of deaths each winter, public health experts say.
The UK Public Health Association says two-tier tariffs - where the first units cost more - penalise the poorest.
It wants the "iniquitous" price system changed so cheaper units come first.
Energy firms say this would hit many vulnerable people who are at home all day and that they spent £150m helping vulnerable customers last year.
Every year the cold weather brings illness and death. Across the UK each winter there are, on average, more than 30,000 fatalities caused by low temperatures.
Some are from falls and road accidents. But most are caused by heart attacks, chest infections and strokes.
UK Public Health Association chairman Professor John Ashton says winter death rates are much higher in Britain than in Scandinavia and should be a matter of of shame.
"What's happening in a lot of these houses is that you'll have an elderly person, perhaps a widow on their own on a low pension, struggling to keep the house warm," he said.
"She'll keep one room warm and then at bedtime she'll go up to her bedroom which is cold. She'll get chilled, and then she'll get a chest infection, go on to get pneumonia and that's it."
Domestic fuel costs have risen sharply in recent years, but Prof Ashton says the problem is compounded by the two-tier tariff used by most energy companies, with a high initial unit cost which falls when more energy is used.
"We spend a lot of effort trying to prevent premature deaths at all ages but particularly in the elderly.
"This is something that we ought to be able to do quite simply, and one of the things is addressing this iniquitous pricing structure."
The lethal impact of the recent cold snap is already emerging in death figures. In Cumbria there were nearly 200 extra deaths in just over a month over Christmas and the New Year.
'Difficult to understand'
Marianne Hornby lives on her own near Egremont. She is on benefits so money is tight and she struggles to keep warm.
"I can't cut down my bill for my food any more than what I am doing. I'm already on a very low level so I try not to think about it because otherwise I just wouldn't sleep. I just try and do a day at a time."
She was surprised to discover that as a low energy user, a lot of her consumption is charged at the higher tariff.
"They're very difficult to understand these bills. But I was quite shocked when I found that. I can't afford to do that".
Prof Ashton says the tariff should be turned on its head - with a low energy price to cover the basics, and then more expensive for people who use more. He says this approach would also discourage waste.
The charity Age UK has backed the idea, as has the environmental campaign group Friends of the Earth.
But Christine McGourty from Energy UK, which represents energy providers, said restructuring the tariff in this way could be complicated, expensive, and may have no overall benefit for those it is supposed to help.
"Many of those higher users are using a lot of energy for a reason. They are families who are at home a lot through the day with children.
"They need to heat their homes and they need to keep warm. That's why they're using a lot of energy. And for those people, changing it would make things a lot worse.
She said last year the energy industry spent £150m on measures to help their most vulnerable customers, including special discounts, rebates and free insulation.