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 117 volts and to shut off when the charge exceeds 143 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.