[Piwoslaw]

Unless you live in a place where it rains often, then this may not be cost-effective. The output would not be kilowatts, but only a few watts. How often does it pour, and for how long (answer may vary denpending on where you live)? Most rain is not more than a drizzle for hours at a time, and there is not much energy in a gutter that is just dripping, probably not enough to overcome internal friction. Collecting water on the roof until there is a certain amount and then letting it rush down onto the turbine may be better, but holding a ton of water on the roof isn't a good idea.

On the other hand, maybe there is a better way to it? Maybe a formula for how much power is to be had per roof area, where the inputs are volume of water per area unit per time unit (how intense) and head of water (height difference between roof and turbine). Very interesting... Now I have something to think about