But Labour said there was no guarantee that the cap would stop energy prices going up next year, while energy companies said the move would distort the market and undermine moves to improve choice.
"Ed Miliband suggested a freeze on energy prices that would have frozen them so that people paying above the odds would have continued to pay above the odds and crucially the prices couldn't have gone down".
May 9 Britain's energy market is inefficient and gives the biggest players too much power to overcharge customers, Energy Secretary Greg Clark said on Tuesday as he set out plans to cap tariffs.
Speaking to CommonSpace, head of policy at Common Weal Ben Wray, said: "A cap on energy prices is at least some government intervention into an energy market rigged against consumers by the big six corporations, although the cap will be set by the regulator not the government which may be a get out for Theresa May if it does not reduce energy bills in practise".
For the Liberal Democrats, former energy secretary Sir Ed Davey said: "It is never a good idea to copy the economic strategy of Ed Miliband".
What do consumer groups think about a cap?
"When you know that some of the most vulnerable people in society are being charged collectively £1.4bn more than the authorities say is justified you have a duty to act on that - and we will".
Former Labour leader Ed Miliband, who promised a price freeze in 2013 and has accused the Conservatives of hypocrisy for adopting a policy they previously attacked as "Marxist", said the Tories could not promise voters their bills would not rise under the cap.
"The CMA's thorough two-year investigation identified low levels of consumers switching energy providers as a challenge, and put forward a range of recommendations to address this", said CBI deputy director general Josh Hardie.
Harman Kardon's Cortana Speaker Revealed
As for the features, the Invoke speaker is powered by Harman Kardon's legacy audio technology and projects sound at 360 degrees. The perforated cylindrical " Invoke " looks similar to the Amazon Echo device, aimed at a similar target market.
In the first major policy announcement of the Conservative campaign, the Prime Minister said her Government would limit the standard tariffs paid by seven in ten families.
"It is important that these measures bed in before looking to further interventions".
It said: " Energy policy must focus on delivering the lowest cost options available today for meeting the goals of affordability, energy security and climate change".
"Further intervention risks undermining so numerous positive changes we are seeing in the market which are delivering benefits for consumers".
If standard variable tariffs milk customers, the picture for consumers on pre-paid energy meters is not much rosier.
Shares in Centrica have fallen 15% since the start of this year and some are speculating that it may cut its dividend to shareholders to cope with an estimated £200 million hit to profits from the cap. With the Conservatives nearly certain to win a majority, it's highly likely the pledge will be carried out.
And price comparison site uSwitch.com said it was a "red herring" which would "kill competition, push up energy prices and leave consumers worse off". "I expect (this) to save families on poor value tariffs as much as 100 pounds".
"It is part of my determination to stand up for Sun readers and build a country that works for everyone, not just the privileged few".
Prime Minister Theresa May confirmed the pledge on Tuesday ahead of publication of her ruling her party's manifesto next week.