I had a guy knock on the door abour installing solar panels on my roof, which sounds like a good idea and something I had been thinking about.

Does anybody have any experience or knowledge of SunEdison installing solar panels for free and then charging you a small amount (3p-8p per kw/hour) for your energy use for 20 years?

It sounds like a good idea, but is there a catch? (I'm always cautious about door to door sales)
Throwing it open to the collective wisdom of SE23.