Well, I have no real-world experience in hydro. However, as with wind and pv, you got it right when you said you need to figure out how much head/flow you have. [For wind, it would be terrain, wind records, and how much tower/generator you can afford; with pv, how much solar exposure you have, etc., and so on.] Until you [we] know that, it is difficult to offer any realistic and useful opinions on this. However, even not knowing, you will likely not save any money in what you are proposing at the start. You may not break even, if you are basing things on how much grid energy you offset or save. But it isn't a reason to forget it, just get your info' together. It may be that you can put in a system that will pay off. And, if it were me, it might be worth it anyway just to have a source of your own electricity even if you don't break even on the cost for a while.