Hi Michael, your basic idea is good, however the amount of water over the dam (or down the rain spout), the distance it falls downwards (its' speed); the diameter of the spout (flume); and the efficiency of whichever device you have to generate the electricity all determine the amount (power in watts of electricity) that will be generated. I would take a guess that the down-spout on your home is probably 3 to 4 inches in diameter and the fall downwards from a 2 story roof is about 16 feet. Depending on the generating device you installed which the water would spin to generate the electricity, I would guess there would not be enough to light a 10 watt bulb with direct current. Keep in mind also that unless you live in a rain forrest, your electricity would be dependant upon the passing rain cloud. As I said, the basic idea is good, but you need 1. a LOT of water; 2. a LARGE fall from the roof to your generator, and 3. an efficient generating device in the spout.
