But he sidestepped questions about whether the energy firms would hike prices in the months between now and the 2015 contest in order to avoid the impact of the policy.
He accused the energy giants of "profiteering" and dismissed claims that they would not invest in new infrastructure, potentially resulting in blackouts, as "scare stories".
Asked on BBC1 consumer show Watchdog to promise the lights would not go out as a result of his policy he said: "Absolutely. And when the companies tell those scare stories - and they will do that because they want to keep a grip in the next 18 months, they don't want me to win power and freeze people's bills - when they say that, it's a scare story and people should know that."
Presenter Anne Robinson asked if he thought the energy firms were profiteering, and he said: "Yes. I think what is happening is they are taking advantage of a market that's not working.
"I think families at home will know what happens: when wholesale prices go up, you end up paying more and when wholesale prices go down you still end up paying more.
But put to him that there was nothing to stop bills going up now in advance of a potential freeze he said: "I want to get into government as soon as possible to bring in that price freeze.
"We are going to bring in not just a freeze but a regulator that can cut prices if the energy companies are misbehaving."
He added: "They have already announced rises even before my speech but that's why I want to get into government as quickly as possible to clamp down, to take action ... I'm going to make sure this is a real and meaningful freeze."
He said the companies were spreading "scare stories about how it's going to affect investment" but they were "unreliable witnesses".
Mr Miliband, a former energy and climate change secretary, also claimed that so-called "green taxes" were being used as an excuse for rising bills.
"Actually the vast majority of the increase that we have seen in people's bills is not to do with that it's because the companies are taking advantage of what's happening."
He said the way to control costs was by freezing prices and creating a regulator "with teeth" for the market.
Mr Miliband clashed with David Cameron over his policy at the first session of Prime Minister's Questions since he made the pledge in his speech to the Labour Party conference.
Mr Cameron claimed it was a "gimmick" because the Labour leader was in favour of green measures that would add £125 to household bills.
Labour said o nly one in 10 households will be eligible for assistance under Government plans to force energy companies to move customers on to their lowest tariffs.
Legislation going through Parliament would give Energy Secretary Ed Davey the power to require companies to switch households on to cheaper tariffs if they are currently on "closed tariffs", which are no longer being offered to new customers.
But figures obtained from the "Big Six" gas and electricity suppliers by shadow energy secretary Caroline Flint suggest that closed tariffs account for only about 2,530,000 of the 27 million households receiving energy nationwide.
British Gas told Labour that it had no customers at all on closed or dead tariffs, while npower had just 28,000. Scottish Power had 100,000 customer accounts on the tariffs, EDF 113,000, E.On 489,000 and SSE 1.8 million.
At Prime Minister's Questions Mr Miliband said the Government's policy would not help 90% of the population, whereas his policy would benefit 27 million households and 2.4 million businesses across the country.
But a Department for Energy and Climate Change spokesman said: "These figures are misleading. The reforms are yet to kick in, and the numbers will change dramatically when they do.
"Before the end of the year, energy suppliers will be forced to offer no more than four core tariffs per fuel.
"As these changes take effect, more people will be on tariffs closed to new customers, and suppliers will need to move these customers on to the best deal for their preferences.
"Suppliers will also be required to tell all their own customers if there is a cheaper tariff for them, and people will benefit from clearer information to help them compare and switch suppliers."