Kind of. Free satellite data starts at 10m resolution per pixel. You will not get a clear signal on small rooftop solar. Same for balcony solar or other near perpendicular installations.

At 10m the only thing you can reliably detect is large scale deployments. If you want to do better than that you need paid data which is prohibitively expensive at large scales.

You can do it quite reliably from aerial imagery at ~7cm resolution or so. For urban areas (if you have access to the data) you can pull out the type of panel and its size and get a reasonably good estimate on power output.

Also doable maybe a bit less reliably with high res satellite (30cm res).

Depending on how remote the area the data can be a bit out of date (years even)

>> Solar must have a direct line of sight to the sun

A popular misconception. Solar panels can generate significant power during overcast clouds (~15% of rated capacity). I have seen panels generate power in overcast conditions when they would otherwise lack any direct line to the sun (high latitude, winter, forest/mountains).

can't you just track solar panel sales?

That is false. Temperature moderation is an important issue and in hot locations solar efficiency may be maximized when trees provide partial shade.