[Sailmc]

Multiply the amps of the water heater you are interested in by 120 volts. This will give you the wattage the water heater requires. If this number is less than 3500 watts your inverter will work. You then need to multiply the water heater amperage by a factor of ten. This will be the amperage draw on your batteries for however long it take to satisfy the thermostadt on your water heater. This will be a sustantial portion of your available amp hours necesitating engine or generator run time. If you know how long the water heater needs to run and the output of your alternators you can determine how much run time will be needed to replace the power used by the water heater.