en.wikipedia.org/wiki/Shockley–Queisser_limit

Tl:dr The max theoretical efficiency for a silicon solar cell is estimated to be 29.4% with real world high end modules topping out at 26.8%

26% in a lab is not the same as 26% in the field after 5 years, but it is nonetheless a good result.

Solar scales out well, so low conversion effiency isn’t really what determines whether an installation is feasible or not.

If you’re evaluating cheap 20% efficient panels vs expensive 25% efficiency panels, you need only scale up the area of cheap panels by 25% to match output. Plus, panels that start at a lower efficiency experience less derating over time, so the difference in performance diminishes as an installation ages.