Mistakes by energy suppliers led to nearly four million customers being overcharged by a total £270m last year.
The blunders cost energy consumers £72 each, according to new research from independent price comparison and switching service uSwitch.com.
The mistakes also cost customers their time, with nearly a fifth waiting between one to two months for an issue to be resolved, and 12 per cent waiting two months. Worse still, nearly one in ten who were overcharged are yet to receive a refund.
More than a third of those whose supplier had made an error said the wrong tariff or product details had been applied.
This was followed by providers levying incorrect fees (31 per cent), as well as the wrong meter reading (27 per cent) and inaccurate direct debt amounts (24 per cent).
"It’s unacceptable in this day and age that customers are picking up the cost of suppliers’ mistakes," Claire Osborne, energy expert at uSwitch.com, said.
"Accurate bills are essential if consumers stand any hope of taking control of their energy use and spend. Recent upgrades by some suppliers to billing systems have resulted in teething problems, but today's figures show there's still more for the industry to do."
"We urge customers to always check their bills carefully, and speak immediately to their supplier if they think they have been short-changed. Consumers should also always provide up to date meter readings to avoid estimated bills, and check that the figure they provided has been used."