There are a few ways to determine the voltage of your water heater. The most common way is to check the data plate on the unit. This data plate will have all of the pertinent information about the unit, including the voltage. Another way to determine the voltage of your water heater is to check the wiring diagram. This diagram should be located on or near the unit. If you cannot find either of these, you can always contact the manufacturer of the unit and they will be able to tell you the voltage.