There were no details on how much the cap would save families, but a press release pointed to the difference between the best and the worst deals on the market being as much as £300.
At the time, Miliband's price freeze proposal was dismissed by the Conservatives as "Marxist".
Share prices in Centrica, which owns household energy supplier British Gas, and SSE, have fallen this year since the ruling Conservative Party indicated its intention to set a price cap.
Tweeting at energy secretary Greg Clark this morning, Mr Miliband argued the Tories were offering no clear guarantee that energy prices "won't rise next year".
Speaking earlier to Conservative General Election candidates in York, Mrs May said that last year's investigation by the Competition and Markets Authority had shown consumers were spending £1.4 billion a year more than they would in a truly competitive market.
"We will introduce a comprehensive programme to strengthen rights at work, make sure new jobs are good jobs, and end the race to the bottom in pay, conditions and job security", he added. Unlike Mr Miliband's freeze, which would stop prices falling, her plan would see an independent regulator set maximum prices, reflecting market conditions.
"Evidence from other countries would suggest this will lead to reduced competition and choice, and potentially higher average prices", it said.
China's April exports rise but slow from March
The value of shipments overseas had been forecast to grow 10.0% according to economists polled by The Wall Street Journal. At the same time, China's imports from Russian Federation increased by 30.1 percent to $13.08 billion, the report adds.
"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". However, Conn said Centrica's United Kingdom energy supply business makes up only a quarter of its operating cashflow. "Further intervention risks undermining so numerous positive changes we are seeing in the market which are delivering benefits for consumers", he said.
"Instead of bringing in a price cap, the Government should spend some money raising awareness of switching and leave the mechanics of an increasingly vibrant and competitive market well alone". These more passive consumers remain on standard tariffs with one of the "Big Six" energy suppliers, including 80 per cent of Britain's elderly, disabled and low income households.
"With a cap it would be very hard for the Big Six to generate the kind of profits they have been able to".
The Conservatives have claimed their price cap on standard variable tariffs would save 17 million families around £80 to £90 a year, or as much as £100 in some cases.
CAB Director of Policy and Advocacy James Plunkett commented in a blog: "From the sound of briefings, the plan could be a full cap on the Standard Variable Tariff, the energy deal that two thirds of Britain's households are on". With the Conservatives nearly certain to win a majority, it's highly likely the pledge will be carried out.
"It will be painful for some companies, especially those now taking advantage of customer disengagement, but it will offer consumers a safety net, protecting them from some of the worst practices of the industry whilst still allowing innovative suppliers to compete", he said.