On this final point, Ofgem is examining rules that would see suppliers inform customers before prices are changed.
Mark Todd, co-founder of energyhelpline.com, suggested that doorstep selling could be to blame for misselling and the Which? report found that one in three consumers do not trust suppliers to sell the right tariff.
"Energy companies certainly need to have a look at the way they approach people, and always approach people in a way whereby they’re using some kind of authoritative calculator," he said.
Which? chief executive, Peter Vicary-Smith, called for the government to set minimum standards for tariffs as part of the new Energy Bill, "so that consumers can be confident that they are getting the best deal for their energy".
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