Connect your solar, wind, micro hydro or other renewable energy system to its charge controller with the diversion output examined for voltage.
Select a standard water heater that matches the output voltage, electricity type (alternating or direct current) and plug it into the charge controller.
Close the outlet valve on the body of the water heater and fill the water heater with water through the inlet valve, then close it off as well. When the charge controller receives more power than is demanded from the loads on the system, it will divert electricity to the water heater, dissipating the excess electrical charge into heat stored in the water.