I had the same question and was happy to see an ML article that actually answered this :) FTA "The team trained the machine learning program...by providing it

I had the same question and was happy to see an ML article that actually answered this :) FTA "The team trained the machine learning program...by providing it about 370,000 images... labelled as either having or not having a solar panel present" So it seems like the Stanford team that did the work built a large ground truth dataset