The challenge with putting solar on your roof is that even if it saves you money over the long run, you're essentially pre-paying your utility bill for the next ten to 20 years -- and who has that kind of scratch?
That's why SolarCity exists: to pay for and install those solar panels, and then lease them to you or sell you the power they produce, for less than your current utility bill. Google just dropped $280 million on the company because they think this is such a fantastic and, in their words, "safe" investment.