Ovo, which is set to become the UK's second-largest energy company, has been forced to pay £8.9m by regulators after it overcharged customers.
The firm, which bought SSE's retail business last year, sent inaccurate statements to more than half a million customers.
Meanwhile, some of its customers did not receive a bill at all, the energy watchdog Ofgem said.
The firm agreed a settlement package with Ofgem to dodge a fine.
The money will be paid to vulnerable customers rather than the Treasury.
Despite knowing about the issues, Ovo did not tell Ofgem.
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