Britain's energy firms are hoarding £1.1bn ($1.7bn) of consumers' cash and customers are being urged to reclaim that money.
According to uSwitch.com, some 12m households are in line for refunds after using less energy than they paid for, based on estimated bills that were "way off the mark".
New figures suggest that the average home could reclaim £93 from their supplier, with over one in ten due more than £200.
Overall, as many as half of all UK homes could be owed a refund. uSwitch said the statistics highlight the importance of taking regular meter readings for accurate bills.
Ann Robinson, director of consumer policy at uSwitch.com, said: "Now is the time to check your energy account to see if your supplier owes you money after the winter. Energy bills are often based on estimates rather than actual usage, so they can be way off the mark."
Suppliers are legally obliged to refund credit to consumers if requested.
Earlier this month, the Competition and Markets Authority (CMA) launched an inquiry into the tariffs big energy companies charge after concluding that millions overpay for their energy bills.
The CMA indicated that it could impose a transitional price cap on the highest tariffs until a long-term solution is found.
uSwitch also warned that almost 4m households owe £507m to power firms.
Robinson added: "Many consumers are also in debt with suppliers. It's important to remember that accounts can fluctuate between credit and debt depending on the season. Being in credit can provide a buffer against the colder months when usage typically goes up. The best advice for ensuring that your bills are accurate is to give your supplier regular meter readings."
The UK's "Big Six" suppliers – SSE, British Gas, Npower, EDF, E.on and Scottish Power – have come under increasing pressure to slash bills to better reflect the drop in wholesale energy costs.
Earlier this month British Gas announced it will cut household gas prices by an average of 5%, its second price reduction of 2015.