Billing blunders cost energy customers around £102 million in the last year.
Around 1.3 million householders were overcharged by an average of £79 each – and some are still waiting for a refund, according to uSwitch.
Nearly one in three of those affected said they were charged for an amount that didn’t match the meter readings they had supplied, while a quarter said their bill didn’t add up correctly.
Other billing blunders include suppliers getting direct debit amounts wrong (8%), applying incorrect fees (7%), getting tariff or product details wrong (5%) and muddling up bills with another customer’s bills (5%).
The research found it took 35 days on average to resolve the billing issues and get the money back, although 7% of customers still haven’t had a refund.
One in 10 customers waited between one and two months to get their money back and almost a third (28%) waited more than two months.
According to uSwitch, further mistakes could be going unnoticed as more than a quarter (27%) admitted to not reading their energy bills properly.
Energy Expert Claire Osborne said: “Customers having to pay the price for suppliers’ mistakes is unacceptable. Households are already feeling the pinch from recent energy price rises and having to chase for an average of 35 days to get their money back simply adds insult to injury.
“We want to see companies do much more to make life easier for their customers, accurate bills are the bare minimum they should expect from their energy suppliers.”
The price comparison site is calling on Ofgem to work with suppliers to improve the design of energy bills to give them a better understanding of the contents of their bills.
It suggests bills should clearly show whether they are estimated or based on actual meter readings, whether the customer must take any action after receiving the bill, alert the customer if they are on a standard variable tariff and a prominent reminder the customer could save money by switching tariff or supplier.
Ofem said it has taken “strong action” to address billing failures by suppliers, with energy firms paying out more than £40 million as result of its investigations.
A spokesperson added: “Suppliers are required by Ofgem to treat customers fairly – and if a customer thinks that their supplier has made a billing mistake, they should contact them.
“We are working with suppliers to make bills easier to understand and Ofgem’s rules already require suppliers to make clear on bills whether they are based on a customer’s actual or estimated energy usage.
“Smart meters will address the issue of estimated readings as these consumers will be billed based on their actual energy usage.”