It's kind of ridiculous at this point. I guess the questions are on-topic, but it still seems better suited for a separate site (maybe one that doesn't exist yet, or the off-the-grid one?).
FWIW, I've been cleaning up tags recently and I bumped a bunch of old solar questions (replacing the tag controller, which is ambiguous and should therefore go away entirely), so what you're seeing might be just that.
This is one area where it is hard to set the position of the off topic line. There are clearly questions on solar which are in the realm of the electrical engineering stack exchange, there are questions which are not e.g panel positioning, and then there are those in the middle.
It is my opinion that we should leave this to the wisdom of the community. You are at liberty to vote a question as off topic and suggest migration. With sufficient support this will happen solving the problem.
There are a few topics like that... Stereo systems, car wiring etc.
Grey areas will always abound.
Does it fit under EE? In reality often yes as a subset, but the OP may get better answers from the appropriate specialized forum.
Also, as any good EE knows, there are areas where cross disciple knowledge is required, for example pretty much anything to do with motors you need to use your mechanical engineering training too.
Ultimately, as someone else mentioned, it's up to the wisdom of the community, however the problem with too much segregation is someone searching for something here can miss some pearl in some other forum. Perhaps something SO should look at.
In order to label such things off-topic, we would need explicit rules. There are many areas that overlap with EE and also many Stack Exchange sites with a scope that overlaps:
It isn't obvious where to draw the line.
Migrating posts between SE sites is rarely a good thing to do - most often such questions just die out. Plus it creates work for moderators at the source and destination sites both. I believe the overall trend on SE is to stop migrating posts, but rather to close them instead.
Arduino questions is another example like the solar setups etc, they are mostly on-topic but would perhaps get a better reception on another site.
Given that the question is actually on-topic here, the best thing to do is to leave a comment to the OP, telling them that their question is better suited for site x. That way we can direct traffic to other sites in a positive way and without creating more work for the moderators.