The payoff on a solar electric/PV system will vary depending on whether it is a commercial or residential installation, installation difficulty, and roof exposure.
A residential system will qualify for a 30% Federal Tax Credit and this will offset a good chunk of the installation costs. Residential systems will show a payback after 7-10 years depending on system cost and roof exposure.
A commercial roof also qualifies for the 30% Federal Tax Credit, the MACRS bonus depreciation, and this helps accelerate the payback periods. A commercial roof can pay for itself in 4-7 years depending on installation cost and solar production. When focusing on the rate of return, a solar electric system will compete and often exceed the rate of return on most stocks or bonds.