> I know it can be done because I asked an electrician. But I dropped the idea when he said it could cost a lot (if done by a professional).

If you have to ask, this is absolutely not the sort of work you should do yourself. Use a licensed electrician.

Definitely true for my primary house. I agree with you, just curious about the technology just in case I need to do something similar to my future cabin -- for example solar power + generator switch -- I guess the the principle is the same.