Testing a mini turbine hydro generator from eBay and then opening it up and taking a look inside.
This little water turbine generator is pretty common on eBay and is available with various output voltages or an unregulated version where the voltage can spike as high as 80 volts provided the water pressure is high enough.
I opted for the 5 volt version thinking I could use it to power USB devices and charge USB power banks. I expected to get less than 10 watts but didn’t expect to top out at around 0.3 Watts!
I removed my shower faucet and screwed the generator in place, a fairly high water pressure test. I doubt you’d be able to get higher pressure without using specialized equipment and certainly wouldn’t get that high a pressure from a stream or river, even if you built a mini dam.
For those with a good stream or river, you might want to consider modifying an old washing machine motor to act as a generator.