With energy secretary Chris Huhne promising to crack down on energy suppliers' complex tariffs, research shows cheap electricity customers find it hard to calculate their bills.
A survey by Which? asked a number of people to work out their domestic energy bill using only the information displayed on their supplier's website and just one of the 36 was able to do it correctly.
Richard Lloyd, executive director of Which?, commented: "People tell us they want to be able to check they're paying the right price for their energy.
"There are straightforward ways that consumers can cut their bills - for example by switching to online deals or paying by direct debit. But that won't help people to pick the best tariff for them."
Mr Lloyd added that the consumer group wants Ofgem to eliminate excessively complex tariffs "across the board".
Meanwhile, research from energy comparison site energyhelpline.com revealed that the Treasury is set to receive a VAT windfall of £197 million as a result of the recent price hikes implemented by the Big Six.
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