5 companies gave a quote without visiting the house.
Shady solar tech sellers mislead consumers to get sales, a new Which? undercover investigation found.
Ten out of 14 solar thermal technology installers exaggerated how much homeowners could save by generating their own heat.
Which? Chief Exec Peter Vicary-Smith said: "Most of the firms in our investigation behaved like true cowboys -they promised huge savings that bore no relation to reality and some really piled pressure on the homeowner to sign up immediately or risk losing a one off 'special offer'.
"The solar industry is too important to our long term energy needs for things to drag on like this. It needs to clean up its act, and if it won't, the OFT and the government will have to step in."
Ideal Solar Energy and Everest potentially broke the law with their dodgy sales tactics.
Savings calculated by Which? independent experts compared to solar installers' quotes*
Independent quote Sales quotes from solar installation firms
Amount solar would cut from Everest 43% Which? household annual gas bill 10% Solar Energy 50%
Amount home owner would save with solar thermal tech over 20 years £5,000 max Everest £35,000
* All quotes were given for the same Which? house used in this study in early 2010.
It's an offence for a company to make misleading or false statements that cause consumers to buy products and to deprive people of the time needed to make a careful choice.
The trading standards officers involved in this investigation felt that the precise nature of Everest's figure made the claim appear carefully calculated and so even more misleading.
Watch secretly-filmed footage of these dodgy sales people in action during the investigation.
Which? is sending its report
to solar trade bodies and the DECC.
Last year the OFT got 1,000 complaints about the solar industry. This is quite high, especially considering there are just 100,000 such installations on UK homes. This prompted the Which? investigation.
Which? found the only company worthy of mention for its helpful manner and sensible advice was Southern Solar.
The Which? study was carried out in early 2010 on a property in south-east England. All visits were filmed and calls recorded. Which? got a solar heat expert and a trading standards officer to check the results. For more details see the May 2010 edition of Which? magazine.
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