Nearly 13 million households are owed a total of £1.7billion by energy suppliers, it is claimed.
The amount, amassed by customers who pay by monthly direct debit, is 13.5% higher than at the same point a year ago, according to Uswitch.com.
Households paying for energy by direct debit often find themselves in credit with suppliers if the monthly payments are more than their usage.
According to the comparison site, it means they should be in credit after summer due to lower usage, and in debt in winter.
But a survey found that 46% of households are still due a refund coming out of winter.
The average amount owed to them is £136, up £10 on last year.
But it says energy firms do not automatically issue refunds, meaning money owed can go unclaimed for months.
By comparison the poll suggests 3.9 million households owe £548million after winter. The average debt of £142 is up £20 on last year.
Sarah Broomfield, of Uswitch.com, said: “Many people are finding finances squeezed as well as using extra gas and electricity because they have to stay at home.
“It will be welcome news for those sitting on unclaimed credit from their supplier.”
