Hundreds of thousands of people are still getting estimated or inaccurate energy bills, the energy watchdog has told the BBC.
More than a third of bills last year were said to be estimates
Energywatch says it has dealt with more than 50,000 complaints about bills in the past year.
More than a year ago firms in the industry were told to improve the way customers were invoiced.
Next week the government is launching a consultation exercise aimed at improving bills and metering.
Complainants say the bills can be wrong, badly estimated or confusing.
One solution could be a meter which displays in detail how much energy a customer is using and how much it costs.
But installing one would be expensive, and it is not yet clear who would bear the cost.
According to Energywatch, billing is the biggest cause of complaints about suppliers.
It said the industry paid out £2.5m in compensation to customers last year.
Of the 200 million bills sent out in 2005, more than a third were estimates.
"With really sky-rocketing prices, some estimated bills that are far too high mean consumers are greatly over-paying and the companies are building up, quite a few times, quite large cash sums that really belong to consumers," said Energywatch chief executive Allan Asher.
The industry insists there have been improvements with a new code of practice and ombudsman to look at serious complaints.
Duncan Sedgwick from the Energy Retail Association said the number of complaints about billing has been "pretty static" over the last three years.