Believe you may be among the 3.5 million households or 300,000 businesses owed part of £400m from the Big Six? Here's how to claim it back.
Ofgem tells ITV News power networks could face fines if it is proved that they failed to respond quickly enough to reconnect customers.
With questions looming over high prices and why it took so long to restore power over Christmas, there will be no respite for the 'Big Six'.
Why are the energy firms paying interest on this pile of customer cash?
We've discovered there is a gap in the rules, with no legal obligation on energy firms to pay interest on all this money they owe customers.
British Gas does not pay interest. The company said they use it to offset other customer debts.
E.ON does not pay interest either. They told us: "Normally only balance owed is refunded”.
Npower, EDF and SSE all do not pay interest.
We raised this with the industry watchdog Ofgem who told us it would expect firms to pay interest if they profited from having the money.
Fines will be issued to any of the Big Six energy companies which fail to return money to former customers, a Government source has told ITV News Consumer Editor Chris Choi.
Energy: Firms have £400m of customer money they are not entitled to they should "expect fines" government source tells me
Energy regulator Ofgem has written to the leading providers calling on them to refund the more than £400 million found to be held in closed accounts.
One of the Big Six energy suppliers has said it writes off four times as much "bad debt" every year as the funds it retains from closed customer accounts.
A statement from SSE said it takes "all reasonable steps" to return money to customers and always views "any credit as theirs", adding:
No matter how much time passes, if they come forward and we can identify an outstanding credit, we will return it to them.
We're happy to work with anyone who has legitimate suggestions for how we can find customers more easily and efficiently.
However, the reality is that the bad debt we have had to write off in recent years is about four times the amount of credit that couldn't be returned, so this is not a situation that benefits us.
Energy companies are commonly owed much more money by people who leave unpaid debts behind them than the surplus that is left in accounts, a spokesman for the industry's trade association Energy UK has said.
He said suppliers agreed with ministers last year they would take "all reasonable steps to trace customers who leave a credit balance behind" but often face stumbling blocks because of a lack of account details.
– Energy UK spokesman
Although companies are working to return money where there are credit balances sometimes former domestic customers provide no new contact details so suppliers don't know how, or to whom, they can return the money.
The most common reasons energy companies end up holding funds are when the bill payer has moved home or when a customer dies and suppliers have no record of the next of kin.
Energy regulator Ofgem has said it "expects suppliers to do more" to ensure that money held in credit from closed accounts is returned to former customers.
Ofgem said an inquiry into the suppliers had found "an unacceptably large amount of money being retained rather than returned to consumers".
Most of the major suppliers, who are known as the Big Six, have recently pledged to automatically refund surplus money to existing direct debit customers.
Ofgem has said it expects the companies to do all they can to return money to individual consumers and to inform current consumers clearly what to do when closing an account.
Ofgem interim chief executive Andrew Wright said:
When many people are struggling to make ends meet, it is vital that energy companies do the right thing and do all they can to return this money and restore consumer trust.
We want to see decisive action by suppliers, individually and collectively, to address this issue and, wherever possible, to ensure that the balances they currently hold are returned to consumers.
Ofgem will today call on the Big Six energy companies to hand back more than £400m that remains in accounts closed by their former customers.
Around 3.5 million households and 300,000 business accounts are believe to be affected by the issue.
The Big Six - Centrica's British Gas, EDF Energy, Eon, Npower, Scottish Power and SSE - supply 98% of UK homes and have all increased their prices in recent months.
Ofgem chief executive Andrew Wright is understood to have written to the companies telling them to give customers whose accounts were not shut down properly their money back.
He wants any funds that cannot be returned to be used to reduce bills for existing customers.
Energy suppliers cannot just blame "old and draughty" houses for rising fuel bills, a leading comparison site has said.
The chief executive of npower earlier claimed gas and electricity prices were rising because British houses waste so much energy.
"The reason why UK energy bills are high is both that the housing stock is not energy efficient but also that suppliers have kept whacking up prices year after year," Mark Todd, co-founder of switching website energyhelpline.com, said.
"Suppliers have raised prices by 140% in the last nine years while users have cut usage. Typical gas usage is down 34% and electricity consumption by 3% and this is in part because many customers can no longer afford to heat their homes.
"The biggest reason for customers cutting usage is unaffordability so without doubt UK homes are now colder than they were in the past. It's crunch time for the energy market and suppliers can't just blame the houses."
Labour's planned freeze on energy prices would hamper investment, the chief executive of npower has said.
"I think it is already having a dramatic effect already. What it's doing is freezing investment," Paul Massard told BBC Radio 4's Today programme.
Mr Massara, who earlier claimed 'Old and draughty houses' were to blame for the UK's rising energy bills, also criticised the Government's plans to reform green levies.
"They made some technical changes on what carbon we actually have to deliver. That reduced the bills by about £53," he said.
"We don't want them to scrap it because actually we think that insulating old people's homes and insulating homes which are leaky is the right thing to do.
"What they should have done is taken all of it into taxation because actually it is unfair that everybody gets hit by the same way in the terms of insulating people's homes."
The chief executive of npower has said he wants to dispel myths about the energy industry in a bid to restore trust in the under-fire sector.
Mr Massara said the actual unit price of energy in the UK was one of the lowest in Europe, but claimed bills are high because British houses waste so much energy.
In the supplier's second "Energy Explained" report, the chief executive said: "At times during the debate on energy, facts have been in short supply, but we urgently need to dispel some myths to restore trust in the energy industry.
"Suppliers control less than 20% of a bill and I want to shine a light on all the different aspects of energy, particularly to reassure my customers that there is no hidden profit. We made a 3.2% margin in our retail business in the first nine months of 2013.
"Over the same period our power stations were struggling to recoup the hundreds of millions of pounds in investment required to build them, and made a loss of £59 million."
Earlier, industry regulator Ofgem accused npower of "misleading" consumers with "incorrect" data on network costs.