The government has yet to clarify whether the cap will be an absolute cap - meaning consumers can not be charged more than a set amount - or a relative cap, whereby the standard variable tariff must not be more expensive relative to fixed tariffs.
Her announcement last week that the plan would go ahead initially wiped more than 900 million pounds ($1.2 billion) off the value of the two British listed companies Centrica and SSE alone.
The Prime Minister pledged to cap energy prices during her Conservative party conference speech, saying she would bring an end "to rip off energy prices once and for all". This is equivalent to 18 million accounts, although four million customers with pre-payment meters and are already protected by a price cap. Ofgem will set the terms.
FILE PHOTO: A gas cooker is seen in Boroughbridge, Britain, November 13, 2012.
Business and Energy Secretary Greg Clark said that customers of the big six energy suppliers were "overpaying to the tune of £1.4bn a year".
Around two-thirds of all energy customers in the United Kingdom are now on these variable tariffs.
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It has also urged suppliers to help move customers onto better value deals.
The Government's proposed bill to provide price protection to those who remain on poor value default deals, such as the standard variable tariff, will give these households peace of mind about the price they pay for their energy.
Ofgem chief executive Dermot Nolan said he did not know how long it would take for the government bill to become law, but it would then take around another five months before the regulator could start capping people's bills. May has previously indicated a cap could cut average energy bills by around 100 pounds ($132) a year.
They argue intervention in the market does not promote competition.
The regulator said it will work on extending price protection to at least 2m more vulnerable households for winter next year, once the timing of the government's price cap is put in place.
Ofgem said on Wednesday that whatever was contained in the upcoming legislation, the government's price cap would not come into effect for the upcoming winter.