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.
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.
More top news
A woman died as she crossed the road when she was struck by four separate motorists who police believe did not stop.
Four people have been arrested for murder after an elderly man, who was viciously beaten and robbed in his own home died.
The British glamour model was lured to Milan before being bundled into a bag and kidnapped for six days, authorities say.