Ed Miliband switched his energy supplier to one of the "minnows" challenging the Big Six providers after being stung by a bill for £1,000.
The Labour leader told the Mail on Sunday that he switched from E.On to First Utility, but does not yet know how much money he has saved.
Mr Miliband has put energy prices at the heart of his assault on the coalition Government, pledging a 20-month freeze if he comes to power.
And First Utility followed his lead by announcing its own freeze until the end of the winter just days later, while giants like the Centrica-owned British Gas, SSE and ScottishPower hiked bills by between 8.2% and 11.2%.
Prime Minister David Cameron branded Mr Miliband's freeze "the politics of the conman" in the House of Commons last week.
But the Labour leader told the Mail on Sunday: " It's a sure sign that someone is losing the argument when they lose their cool.
"He has gone from saying it wasn't a problem to telling people to wear jumpers. For goodness' sake, it's an insult. It's Edwina Currie-esque.
"He used to say he was a Prime Minister for the centre ground, now he's a Prime Minister for Centrica. He's not willing to stand up to the big energy firms."
Downing Street insists that Mr Cameron has not advised people to wear jumpers to cut their heating bills, though a Number 10 spokesman did tell reporters that people "may wish to consider" wrapping up warm.
Mr Miliband was buoyed by last week's intervention from Conservative former prime minister Sir John Major, who called for a windfall tax on the power firms.
"The Tories want to re-run the 1992 e lection, but now they can't," said the Labour leader. "What John Major was saying is that Cameron has lost touch with millions of the people who voted Tory in 1992, whom I call 'Major's millions'.
"And we are the people who are talking about issues that really matter."
Mr Miliband said the decision to switch energy supplier last winter was taken jointly with wife Justine. Asked how much they had saved, he said: "We don't know yet. We had a huge big bill for last winter - as much as a thousand pounds."
The Labour leader said he would be "quite happy" to pay the mansion tax on properties over £2 million backed by his party and the Liberal Democrats, which is likely to catch his own London home.
And after Deputy Prime Minister Nick Clegg spoke out publicly against Conservative policies on free schools and immigration, Mr Miliband suggested that the Lib Dem leader may be shifting in Labour's direction, saying: "Maybe he has moved towards us."