To me, that looks like a sigmoid that has passed its peak. But, assuming it’s an exponential, it doubled between 1975 and 2015 (i.e. 40 years).

Here’s some graphs for PV: https://en.wikipedia.org/wiki/Growth_of_photovoltaics

PV is doubling every 3 to 3.5 years.

3 doublings of PV at that rate gets us to about 9-10 TW peak. I don’t know the actual average global capacity factor, but I do know the current global electricity usage is only about 3 TW: https://en.wikipedia.org/wiki/List_of_countries_by_electrici...