Billing blunders by energy firms cost customers more than £102m last year, figures reveal.
A survey by energy switching site uSwitch found around 1.3 million households were overcharged by an average of £79 each by suppliers – and some are still waiting for a refund.
One in three of those affected said their bills did not match their meter readings, while a quarter said the amount they were charged did not add up correctly.
Other mistakes included households receiving someone else’s bill, incorrect fees being charged and energy suppliers getting direct debit amounts wrong.
It took customers who had been billed more than they should have been an average of 35 days to get the money back.
Around a third had to wait more than two months, while 7% are either still waiting or have given up.
The total cost of errors may even be greater with more than a quarter of customers admitting to not examining their bills properly – if at all.
Claire Osborne, energy expert at uSwitch, said the mistakes were “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,” she said.
“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.”
Uswitch called for a crackdown by industry regulator Ofgem, demanding suppliers reimburse customers within a month and make bills easier to understand to show whether they are based on an actual meter reading, if the account is in credit, and to include reminders that the customer could save money by switching tariff or supplier.
An Ofgem spokesman said: “We have taken strong action to address billing failures by suppliers, with suppliers paying out over £40m as a result of our investigations into these and related customer service issues in the last two years, and we continue to monitor the market.
“Suppliers are required by Ofgem to treat their customers fairly – 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.”