Do that math and take into account how the output of solar panels decreases with age. I did the math once with the state subsidies we had and was leery with the payoff period being between 7-12 years. I guess do it if you want to be green, though the manufacture process for the panel isn't exactly environmentally friendly. If you are doing it for money reasons, you would probably be better off and taking less risk if you took that large chunk of money you would have used to pay for your panels and just invested it into the market.
Jbp8653 payoff would be 350-17 = 333. (35000*70%)/333 = 5.83 or about 6 years at minimum. 275-109 = 166. (35000*70%)/166 = 12.2 years. This is all not including loss in efficiencies as the panels age but you are look at something that will be greater than 6 years and up to something like 12.