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Almost 13 million UK households are owed a total of £1.8 billion by their energy supplier, according to new Uswitch research.
Energy customers who pay for their bills by direct debit can often find their accounts in credit as their monthly payments don't exactly match their energy usage, which can fluctuate throughout the year.
Because most people use less energy in the summer and more in the winter, households should be in credit in the warmer months and in debt during colder weather - ideally balancing out along the way.
But coming out of winter this year, almost half of UK households (45%) are due a refund from their energy supplier.
The average amount owed to customers is £142, a £6 increase compared to the same period last year. A quarter of those in credit are owed more than £200, while more than half a million could claim back credit of more than £500.
Not all energy suppliers automatically refund credit balances - 59% of bill-payers say their supplier has never automatically credited their account and more than a third (35%) say their energy provider has never contacted them to review their dircet debit amount.
Sarah Broomfield, energy expert at Uswitch, said: "At a time when many people's finances are stretched any windfall would be gratefully received."
"Many people who have been affected financially by the pandemic may be looking for ways to save money, and it's worth checking with your supplier to see if you are owed any money following your most recent bill being paid.
Almost half of energy customers (49%) said they don't know how to claim back their credit balance. The process can vary between suppliers, so it's best to contact your gas and electricity provider directly.
However, this could become easier in the future. Energy regulator Ofgem is currently consulting on a automated system which would refund direct debit customers on the anniversary of their tariff starting.
Sarah Broomfield said: "It's clear that Ofgem's proposal to introduce automatic rebates will benefit a huge number of consumers, particularly those who do not know how much credit they have, or do not know how to obtain a refund.
"It's also important to provide regular meter readings to your energy supplier if you do not have a smart meter. This will make it easier for your supplier to see if you are using less energy than predicted, and they may reduce your direct debit payments.
"If you are out of contract with your supplier, you could also save yourself money by comparing energy and switching to a cheaper deal."