Hell, even something as simple as a water wheel in a little creek diversion would be cool to have as a backup source.
If by " backup source" you're referring to something that adds a trickle charge to your battery bank.
Like Morewood says,
supplement solar during cloudy, inclement days.
You can use multiple sources to charge batteries, I use smaller solar setups and a sailboat style wind generator.
But any single source will have to be rather large to run much by itself
