30 percent roof rule 7

https://whenreplaceroofpost.valiantfield.com/posts/knowing-when-to-replace-a-roof-a-homeowner-s-guide

The 30 percent roof rule is a handy solar guideline: if your roof's south-facing area is less than 30% of your home's total floor space, you might struggle to fit enough panels to offset typical energy use, so you'd need to look at other renewable options or efficiency upgrades first.