How to turn rain water into electricity

You can. But rain dosent last that long. People trap rain water to use for irrigation. But we don't have enough rain to make it work too hard.

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.

Many Millions of cubic yards of water pass thru a hydro-electric generator's driving turbine, just to make about a megawatt or two, and Yes this supplies MANY places with electricity.  The idea of rain water turning a generator is feasible, but the amount of electric current would be not much more than infinitesimal, and would have to be used separate from the home's electrical system, unless generated as DC, used to charge a battery system, that supplied power to an inverter that would convert the DC into the AC that you use in your home.  It takes many cubic feet of water to produce usable amounts of electricity.     

I think all think of Big machines and TURBINEs and lots of water, imagine a HIGH RISE BUILDING approx 40 to 50 floor height, that is 600 to 700 feet high, the rain water collecting on roof top to all possible ways from the complete building outer surfacer, when channeled down from one outlet, with many small turbines in its path, will rotate small turbines generating power, not one turbine but many many turbnines all the way down upto a BIG turbine in basement of building. U imagine 700 feet with 700 turbines small size, and one big at bottom, all together is so much.... i think this should work....

Also in dams, they use one turbine, i always think such huge pressure of water, why dont they just use hundreds of turbines in the way of water, though pressure will be readuced a bit, but GRAVITY is the best powerful thing to make it work... Correct me if i am wrong you guys out there, i might not be sounding gr8....

yeah! definitely a good thinking  but at the same time u hav 2 b work hard to put it into existence

I have tried to put a system like this in place for over a decade now, and it just doesn't work because like previous answers there is just not enough rainfall to produce any significant electrical output. I tried different set ups and the one that produced the best results was to charge a battery pack, but that took 14 weeks over a wet period to fully charge the battery. It cost me around £500 to produce a working prototype, and it produced about 50 pence worth of electricty in 14 weeks so it would take you over 250 years to get your money back. So it would never be cost effective to install.