It was also found that while seven per cent claimed to be on the deals, online one per cent actually are.
Jonathan Stearn, head of campaigns for energywatch, said consumers were being "left in the dark" by energy suppliers.
He said: "They do not know how much has been allocated from renewable sources, nuclear power or fossil fuels.
"We all should be given a fuel mix for the electricity we buy so we can start to understand the claims that suppliers make."
energywatch recently described new plans by the government to help those struggling with fuel poverty as "deeply disappointing".
The organisation's campaigns director, Adam Scorer, said the new measures have "a hollow ring to them".
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