Which? said one advert for Southern Electric (SSE) claiming to make customers up to £100 richer could have actually cost £133. And an E.ON advert offering free energy for two months saved people just £4.
The watchdog monitored energy company adverts for more than a year, concluding that seven could be misleading. While it found most of the claims in the adverts were "strictly speaking" correct, they could be making the deals and offers sound more tempting than they actually were.
The SSE advert promised consumers would be up to £100 richer if they switched to its capped tariff. In a survey, Which? found 80% of people agreed this implied they would be £100 better off and 53% said it appeared to be a good deal.
But Which? said: "Assuming prices don't change, an average user would pay £233 more a year. Even with £100 off, they'd be £133 poorer."
The British Gas Fix and Fall tariff promises people will benefit from any fall in prices but Which? warned: "The tariff was £1309 a year for dual fuel. The cheapest fixed tariff for a similar period (to March 2014) was £222 cheaper."
E.ON said: "We're sorry this advert was confusing. Following feedback we withdrew this tariff."
SSE told the consumer group: "SSE has fair prices and simple products."
British Gas it received no complaints relating to the allegations.