An undercover investigation by consumer magazine Which? suggests companies have been giving misleading advice to people looking to buy solar panels.

The probe found that three quarters of companies overestimated how much energy the solar photovoltaic (PV) panels would produce and most of them underestimated how long it would take for the system to pay for itself.

One company overestimated the profit by £4,275 over 25 years and underestimated the payback time by three years, compared to the calculations made by Which? experts.

Which? also claimed that the Government's rules to work out energy output doesn't take into account key factors like where people live. Seven out of the 12 salespeople visiting the Which? undercover house even recommended installing solar PV panels on a shaded part of the roof and eight companies didn't question customers about how much energy they used.

Speaking about the research Philip Sellwood, chief executive of the Energy Saving Trust, said: "While this study is worrying, it's not entirely unexpected. The findings echo the results of our recent heat pump field trial in finding that one critical aspect of the success of these technologies in the UK is training."

The Which? investigation also found that some customers were also subjected to the hard sell.

Richard Lloyd, Which? executive director, said: "It seems extraordinary that the Government's rules require companies to ignore whether you live in Cornwall or Scotland when working out how long it'll take to pay for the solar panels. It's obvious that the more sun you get, the faster the payback. The Government has to put this right.

"It really isn't difficult - companies just need to make sure they send people who are qualified to ask the right questions, do the right checks and give customers better advice."

The Energy Saving Trust is also working to safeguard customers by discussing with the industry ways to improve installation standards. Mr Sellwood added: "Installers and customers both need access to clear and impartial information, allowing them both to make a balanced and informed choice of the right renewable energy for the right home."