I have not done this, but wouldn't a simple way to do this be to get a new or used electric hot water tank, replace the element with one suitable for your dumped power specifications, and use that through the existing thermostat to preheat the water? I don't know about elements up to 350V, but for low voltage DC, and up through 240V there are elements available at varying voltage intervals, and I wouldn't be surprised to learn that 480V elements were available - might want to check the capabilities of the thermostat though. Since the elements are simple resistive elements the impedance and resistance are equivalent, you can calculate the Ohms resistance of the element, and hence, what the power draw will be when reducing the voltage across it. You would need a second dump for when the water reached its maximum temperature and the thermostat disconnected the element, unless the water heater was sized such that this would never occur. Rich