Why are the energy firms paying interest on this pile of customer cash?
We've discovered there is a gap in the rules, with no legal obligation on energy firms to pay interest on all this money they owe customers.
British Gas does not pay interest. The company said they use it to offset other customer debts.
E.ON does not pay interest either. They told us: "Normally only balance owed is refunded”.
Npower, EDF and SSE all do not pay interest.
We raised this with the industry watchdog Ofgem who told us it would expect firms to pay interest if they profited from having the money.
More top news
People who suffer from back pain may have closer links to their chimpanzee ancestors, according to researchers.
David Cameron will say the Tories are the party of the "grafters, roofers, retailers and plumbers" as he set out plans to help small firms
Lib Dem leader Nick Clegg will say his party are fighting '60 Eastleighs' on the campaign trail in the Hampshire constituency today.