A group of MPs has criticised three energy firms for "back-charging" customers with prepayment token meters.
Powergen, Scottish Power and npower are accused of back-charging nearly 750,000 customers for underpayment after putting prices up.
A total of 131 MPs supported an early day motion which said the practice forces customers into "overnight debt".
Powergen said it was replacing 190,000 token meters with "key" meters which would quickly change to new prices.
The early day motion was put forward by Labour MP Brian Donohue.
The industry watchdog Energywatch said that some householders are put into debt after the firms increased token meter fees to their new rates.
The watchdog's campaigns director Adam Scorer said: "Three energy suppliers are creating instant misery for hundreds of thousands of consumers.
'Ancient meters'
"People with these ancient token meters think they are paying upfront for their electricity only to find themselves in debt when the meter is recalibrated."
Powergen said that its £21m upgrade of its prepayment token meters should be completed by the end of the year.
"During this period of change it is of utmost importance to us that we support our most vulnerable customers," it said in a statement.
"We will always respond to genuine cases of hardship and will deal with each situation sympathetically, on a case-by-case basis.
"In particular, we will agree a repayment rate as low as £1 per week and also provide support through our Caring Energy programme."
The firm added that customers whose meters had not been upgraded by the end of March would not accrue any further debt after that date.
Scottish Power has also said it is replacing all its prepayment meters over the next few years.
Last month a cross-party group of MSPs met the firm to request it stop backdating charges.
Npower has so far not responded to this story.
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