More than half of households are in credit with their energy supplier and could claim up to £1.2 billion between them, according to a study.
More than 13 million homes (52%), are in credit by an average of £86, up from 47% of households last year, price comparison site uSwitch.com said.
More than a fifth (21%) 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.
At the same time, almost four million homes (14%) are in debt to their energy supplier, owing an average £128.
USwitch urged households to take regular meter readings to improve bill accuracy, adding that spring is the best time to reclaim any 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 that they must refund any credit on the request of 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. Reclaiming this credit has never been easier as new rules from Ofgem mean suppliers must refund this money to customers whenever they request it.
"However, those who are in the black should remember that this may only be temporary as 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 way for people to stay on top of this and ensure their bills are as accurate as possible is to take regular meter readings."
YouGov surveyed 2,138 adults online between February 3-6.