90 Percent Rule

I read a news about Tesla Working On 90% Autonomous Car.

It attracted my attention not only because Tesla makes very nice cars, and I also write about future mobility.
I liked Elon Musk's rule of 90 percent. It is very close to my rule which I apply to Performance Homes and integrated energy technology. I insist that we should strive to achieve 90% of efficiency. This is the most cost-effective target. If we want to move from 90% to 99%, we need to be prepared to spend the same amount of effort on top of it, i.e. double it. And what about the 100% efficiency (so called "net-zero" approach) ? You will likely need to triple or even quadruple the effort.