By bizn_fun 
Energy UK, which represents suppliers, said people should check letters and emails and contact their old suppliers if they think they have outstanding credit.
The majority of closed account balances are returned automatically, Energy UK said. But sometimes it is hard to contact a person after they move house and close their account, so customers are being encouraged to check for themselves.
Tim Jarvis, director general for retail at Ofgem, said: “Moving house requires a lot of life admin – and it’s understandable that some things will be missed.
“But with almost two million closed accounts currently in credit, the message is clear – if you’ve moved in the last five years, reach out to your old supplier, provide them with the correct information, and you could be due a refund.”
The advice from Energy UK is to keep your direct debit active until the final bill has been settled. Any leftover credit should automatically be paid to customers.
Under Ofgem rules, suppliers must issue a final bill within six weeks and issue a refund within 10 working days after that, but Ofgem said missing customer details were causing delays.
Earlier this week Ofgem announced plans to clear some of the record levels of debt that customers owe to suppliers.
Energy debt and arrears in England, Wales and Scotland rose by £750m in a year to £4.4bn.
But the proposed plan to clear this debt would involve higher bills for everyone else, a solution that has been criticised.
Most households already typically pay £52 a year to deal with historic debt.
The chair of the government’s Energy Security and Net Zero Select Committee said “parts of the sector that are making healthy profits” should contribute towards the debt.