Credit can build up when customers use less energy than their supplier originally estimated.
And the average household in credit has £102 sitting in their account, according to website Energyhelpline.
Victoria Arrington, a spokeswoman for Energyhelpline said: “This is a secret that needs to be told.
“It’s wrong that energy companies which are raking in billions of pounds a year should be holding consumers money ‘in credit’.”
The website says having a small credit can come in handy for customers who use direct debit as well since it can cover the cost of when usage may be higher than usual such as in the depths of winter.
Still, they believe too many suppliers take more than most customers need.
Industry regulator Ofgem says energy accounts are more likely to be in credit at the end of the summer, and advises households to give their supplier regular meter readings.
It said: “If you’re concerned about the size of your balance, you can ask your energy supplier to refund it to you.
“Suppliers must do so promptly, unless there are reasonable grounds not to.”
There is no limit on the amount of credit you have in your account before you can ask for it back.
Still, customers may notice that credit can suddenly be wiped clean after they give an up to date meter reading.
Trade body Energy UK said: “Energy companies aim to smooth direct debits payments over the course of the year so that customers pay the same during peak-usage winter months as you would during the summer months when often less energy is used.”
This has come at the same time that Ofgem published data showing energy firms have seen the cost of supplying gas and electricity jump 6.8 per cent since the start of August.
Firms have said that it is pivotal to look at the overall balance of the year, as households will use less in summer but more in winter.