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
Speaking to ITV News' Mark Austin on the anniversary of the London bombings, ex-PM says nations around the world will need to respond.
A Canadian man has been arrested after tying more than 100 balloons to a garden chair and flying over the city of Calgary.
A van bearing the faces of some of Britain's most wanted fugitives will be driven around parts of Spain popular with expats in a new push.