This is a useful guide I found for converting your traditional AC hot water heater to DC safely.
This is extremely useful for off-grid setups, where efficiency is king! That’s because batteries and solar panels tend to run at DC, and must otherwise be converted to AC using an inverter in order to produce usable energy in most homes.
DC to DC requires no conversion via an inverter, and is therefore much more efficient. This way your hot water is powered 1:1 by your solar panels and battery. You can even make your hot water or base board DC heater into your dump load, converting energy that would have been otherwise wasted into hot water or hot air.
Convert AC water heater to DC using low voltage
Off-grid Hot Water Heater as Wind Turbine Heat Dump - AC and DC - YouTube
Submersible DC Water Heating Element with Adjustable Thermostat
Charge Controllers | Missouri Wind and Solar