Christophe
This is exactly what I tried to explain a little above on my other post.
If the watercourse cannot give more than 200 watts, an alternator like mine of 500 watts can not rotate at 1600 rpm which gives the maximum possible, so we must use a reduction to have the maximum torque at a lower rpm to optimize the yield otherwise there is blockage.
The advantage of being able to change the ratio is to draw the maximum of watts, you may have the most efficient generator that is if it does not run at its torque / load ratio, you will not get much.
Admittedly the efficiency of an alternator is not the best but for a small chalet where we use it on occasion but I think that for this project, the cost / use ratio allows ample use of an alternator instead of 'invest 300 or 400 euros and risk having the turbine stolen when you are not there.
Note that the difference between an alternator and a magnet generator is not as high as it is claimed, moreover when it is started there is no more need to supply it which increases the efficiency.