EnergySage has cooked up a solid article about comparing solar to other investments that you can either refer prospects to or rip from for sales conversations. They walk thought ROI and ROI calculations and look at how to compare solar to investments like the S&P, CDs and T-bonds. The article also gives a nice, detailed list describing why solar is the superior investment (i.e. assured, increasing returns, protection against inflation, impact on other assets, etc.). They wrap up with a  little overview of financing options.