As an aside to the topic, that's not quite how the maths would work. If the panels were 100% more efficient, you wouldn't need 100% less of them

If the panels were 20% more efficient, you'd want 1 -1/1.20 = 16.7% less of them.

Yes, but that only becomes a win if the new panels cost less than 20% more than existing ones.

If they cost less than 20% more _total_, including installation costs, which is interesting because it means the higher the installation costs in a particular area the more likely it is to be worth the added cost of the more efficient panels (which presumably aren't more difficult to install than traditional panels).