The survey by the price comparison website says the suppliers have made billing errors to more than three million customers and over charged them by an average of £78 each.
Scottish Power has the highest percentage of billing errors (16 per cent) among the large energy suppliers, according to the report.
The company said the increase had been caused by the new IT billing system it introduced last year. It said: “This has been challenging and resulted in service problems for some of our customers as accounts are being brought up to date and bills issued.”
Scottish Power added it has brought in an additional 450 customer service advisors to deal with the problems.
At the other end of the table, Eon and EDF Energy have the least number of billing errors at eight per cent, ahead of British Gas (8.2 per cent), Npower (13.9 per cent) and SSE (14.4 per cent).
Of the energy customers with billing problems, 45 per cent said the problem was caused by a different meter reading than to the one they had supplied.
Other complaints include issuing the wrong tariff or product details (28 per cent) and setting up direct debit at an incorrect amount by their supplier (24 per cent).
Uswitch.com energy expert at Tom Lyon called the results “unacceptable”. “Recent upgrades by some of the big six to billing systems have resulted in some teething problems, but the industry must do more to tackle these issues and reduce errors,” he said.