The energy regulator Ofgem is not doing enough to ensure that energy company profits are transparent, MPs say.
The Energy and Climate Change Committee (ECCC) report said the watchdog was "failing consumers by not taking all possible steps to improve openness".
The committee said that "working out exactly how their profits are made requires forensic accountants".
Ofgem said it had made energy companies produce yearly financial statements and they had been reviewed by accountants.
Sir Robert Smith, on behalf of the committee, said: "At a time when many people are struggling with the rising costs of energy, consumers need reassurance that the profits being made by the 'big six' are not excessive."
The big six are E.ON, SSE, British Gas, npower, EDF and Scottish Power.
They have different divisions to deal with the different functions of their businesses: generation, trading and supply.
The committee said that the divisions sometimes bought and sold services and energy from each other, making it difficult to work out how much money was being made overall.