More than 13 million homes (52 per cent), are in credit by an average of £86, up from 47 per cent of households last year, price comparison site uSwitch.com said.
More than a fifth (21 per cent) are in credit by more than £100, while 260,000 households are potentially able to reclaim more than £500 from their supplier.
The increase was due to the mild winter and households cutting back on their usage because of concerns about the cost of energy, meaning they paid for more gas and electricity than they used, the study found.
USwitch urged households to take regular meter readings to improve bill accuracy, adding that spring is the best time to reclaim money after the high-usage winter period.
The big six energy companies have committed to reviewing all direct debit accounts every 12 months and refunding any credit directly back to customers.
Ofgem has also ruled the companies must refund any credit on request from the customer at any point in the year.
USwitch director of consumer policy Ann Robinson said: "With temperatures rising, now is the time for people to spring clean their energy accounts and find out if they are in credit with their supplier.
"Energy bills are often based on estimations rather than actual usage and so they can be very far off the mark."