Hi Biso,
A system working between 5 to 100 Watts is colloquially known as a 'Micro-hydro system'.
Such systems can come in many forms.
You mention so little about the system you intend to use, that i must make some guesses.
12 volt DC arrangement.
This has much in its favour.
12 volt DC devices and utilities are quite common.
Wiring is usually easily understood.
12 volt lamps are more efficient at providing light than their higher voltage counterparts,
they are also much more sturdy and durable.
Additional supplies can be added in as and when they may become available or needed.
(With AC supplies this is a whole bag of worms and problems for the non-electrically minded.)
You say you would be expecting about 80 Watts of available power.
Again you dont say if this is continual, so i have to guess.
I will guess it is available continually,
so with a couple of decent batteries this should give provision for a couple of hundred watts,
for about a third of the time.
So if used for lighting for a couple of hours in the evening, and maybe an hour or so in the morning,
that 80 Watt system should cope if its left running.
Such small systems are unlikely to be accepted into a grid back-feed arrangement,
where the electricity company will pay for units fed back to them,
because the system is too small to warrant any interest.
The only way this might apply is if your land has legislation compelling the elecricity companies to co-operate and accept such arrangements.
You make no mention of your location.
A 24 volt DC Micro-hydro system is also a good choice, because there are many utilities made for trucks and mobile homes available in 24 volt which are difficult to get in 12 volts.
I am thinking of doing it with pulse width modulation.
If you make the system using 12 or 24 volt generators, you can use ones from vehicles, which have their regulation built-in,
they are normally set to charge lead-acid batteries.
If you wish to use Nickel-Cadmium batteries, then you will have to include special arrangements on the charging side of things. You will have to look this up, its not difficult but it is more involved.
For simplicity i would suggest you stick to 12 Volts DC, use normal lead-acid batteries.
Try to keep the battery or batteries clean and tidy.
Make sure there are adequate fuses in the arrangements, excess current from batteries is well known as a fire hazard.
Best of luck with your setup.
John