I was recently excited about an idea to link up a tiny waterwheel to my downspouts. After a little online reading, I learned from a series of circa year-2008 posts that downspouts would not really produce a reasonable enough amount of power a turbine, but wastewater flow would. It is illegal in the City of Pittsburgh to tap into the wastewater systems, but in places where this is allowed, it seems like a reasonable way to collect from a passive energy source.
Here are some websites for reading:
This is an area where I think community leaders ought to cast a eye while they consider ways to minimize power draw from the overall power grid. If each home could pull from a passive source to generate a low amount of energy (particularly clean energy), it might just add up to something significant.