Experts have called on Liz Truss to correct her claims on TV and radio that no families will have to pay more than £2,500 in energy bills every year as the new cap comes into force on Saturday.
Meanwhile regulator Ofgem has urged people to take a photograph of their energy meters ahead of the change, but said they do not have to submit the readings straight away.
The Prime Minister has repeatedly and incorrectly said no household will pay more than her energy price guarantee over the next year.
But the guarantee is based on how much energy a family uses – and half of all households are likely to face bills of more than £2,500.
Adam Scorer, chief executive of fuel poverty charity National Energy Action, said: “It’s simply not true that bills are capped at £2,500 and it’s vital that people understand that.
“The energy price guarantee is not an all-you-can-heat buffet. If you use more, you will pay more.
“Millions on tight budgets do not fit the ‘average’ tag. They can use more energy than the average because they’ve got medical conditions, larger families or really difficult-to-heat homes. They may face bills much higher than £2,500.”
Money Saving Expert’s Martin Lewis added there is “confusion” over how the cap works.
Under the cap, households will pay 34p for every unit of electricity and 10.3p per unit of gas they use.
The typical household, which contains 2.4 people, uses 2,900 units of electricity and 12,000 of gas according to Ofgem’s calculations.
Only households that use that much energy will see bills of £2,500 – those that use more will pay more, and those that use less will pay less.
On Thursday, fact-checking organisation Full Fact said the “public deserves better” after Ms Truss repeated the false claim on local radio.
It had already called out the Prime Minister after she made the same claim in an interview with US broadcaster CNN.
She said at the time: “What we’ve done is we’ve made sure that no family, no household is having to pay more than £2,500 on those energy bills.”
Full Fact wrote to the Prime Minister to point out her mistake, but she later repeated the falsehood on a local radio round on Thursday morning.
She incorrectly told BBC Radio Leeds: “The action we’ve taken on energy bills will mean that… people living in West Yorkshire aren’t going to be facing energy bills of £6,000, which is what was forecast. Through the energy price guarantee, the maximum will be £2,500.”
Full Fact said: “We wrote to the Prime Minister about getting this wrong only yesterday.
“The Government’s energy plans will affect every household in Britain this winter.
“And yet Liz Truss has repeatedly misled listeners this morning.”
Experts said households have time to submit their meter readings for days after Saturday – if they all do it in one go it could crash energy suppliers’ websites.
Energy bills were set to rise to £3,549 in October for the typical household before the Government’s intervention, which changes that to £2,500 for the next two years.
But despite the help it is still around double what energy bills were a year ago.
People can take action by making sure the flow temperature on their condensing combi boiler is set as low as possible. They can also save by turning down their thermostat if safe, taking shorter showers, and investing in extra insulation.
Experts say appliances which produce heat or cooling are the most expensive to run. This includes ovens, central heating and space heaters, air conditioning and boilers.
Ofgem also urged people to be wary of scammers. A £400 rebate on bills will be automatically taken off bills, so texts purporting to be from the Government should be ignored.
One such fraudulent text read: “GOVUK: You are eligible for a discounted energy bill under the Energy Bills Support Scheme. You can apply here,” providing a link to a website.
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