Energy regulator Ofgem has launched an investigation into Ovo Energy over the accuracy of data supplied to customers.
The probe will examine whether Ovo breached its licence conditions related to giving customers consumption information that is accurate and not misleading, Ofgem said.
It is centred on estimates the supplier gave customers about how much energy they used during winter 2016-17, and the accuracy of the annual consumption figures it issued in annual statements.
Ofgem said in a statement: "Incorrect consumption figures can lead to customers receiving inaccurate bills, annual statements and other important information.
"This in turn can lead to customers being unable to properly manage their accounts and deciding to switch based on incorrect information."
It comes after the head of Ofgem told MPs earlier this month he is not getting a bonus this year as he apologised for moving too slowly to protect vulnerable gas and electricity consumers.
Dermot Nolan was accused at a parliamentary committee hearing of a "passive" approach to his job of regulating the market, which had led to many customers paying higher bills.
The Ofgem chief executive said he regretted not taking swifter action to cap payments by vulnerable people on power companies' standard variable tariffs (SVTs).