If we were looking for a planet like our with 30 billion humans all at a peak standard of living... 9% of the planets land would need to be capturing solar energy.
At our current technology, we can only detect such a planet if it was covered with 23% solar capturing.
Of course, it's a pretty fucking big presumption that our "advanced" energy technologies (solar, nuclear) are the end state of a civilizations energy innovation 😏