I found an article from 2010 saying that the CO2 breakeven for solar PV was at around the 3-year mark. That article mentioned 225 W panels, the state of the art today is 400W with presumably the same CO2 load to produce. So that would put the payback at more like 2 years. It would get worse again to account for the energy losses in the desalination, CO2 collection and conversion.
