It's possible to harness water flow from house or building gutters to power a generator. A number of commercially available components as well as home-brewed devices can be combined to harness water flow for generating electricity.
Water Pressure From Your Gutter System
Route all the building's gutters into one down pipe and narrow the spout, increasing the water's force. The spout should be as low as possible to make use of gravity. Experiment with the opening's size during rain storms. If the gutter backs up, it will lose energy meant to turn your generator wheel.
Water Wheels for Your Generator System
Commercial waterwheels for generating electricity are available. A pre-assembled system has the electrical generator already integrated into the waterwheel. If you have to build one, your goal should be a wheel that generates enough force to turn a 12-volt DC motor when your area's average rainfall is applied.
Connecting the Electrical System
Wire the DC motor to a charge controller. The charge controller should be set to allow current when the charge of your battery bank drops below 11.7 volts and to shut off when the charge exceeds 14.3 volts.
Create a Deep Cycle Battery Bank
Wire the charge controller to a deep-cycle battery bank. Many manuals for this type of project give details on how to calculate the number of batteries you will need.
The Final Component, an AC Inverter
Wire the battery bank to an AC inverter. You can plug household devices directly into it or wire the inverter to your house's electrical system.