There are 12V elements, I think $100-200 buys a 12V/120V combo element.
A 1500W/120V element, original equipment in a lot of marine water heaters, is 
~10 ohms.  At 72 volts this would be 7 amps X 72V or about 500Watts, at 48 V it 
is 48VX4.8A or about 250 Watts.  The resistance may change slightly with 
temperature/voltage.  Either would work.  You might be able to find a 
2000W/120V/~8ohm element.
BUT, if you need to keep a dedicated 48 or 72V battery set and charger, that 
would seem to be the hard way to do  it.
If you had a solar array operating at 48 or 72 volts going to a MPPT 
controller, you could grab some power ahead of the controller, add about 30% to 
the "nominal" (multiple of 12V) voltage for the actual panel operating voltage.
Lee
  ----- Original Message ----- 
  From: [email protected] 
  To: [email protected] 
  Sent: Sunday, March 22, 2009 10:36 AM
  Subject: Re: [Liveaboard] Hot water heater


  So if you had four or six batteries, how functional would using say 48 or 72 
volts DC to heat the water? Or, does anyone make a heating element that would 
be sized appropriately for 12 volts?


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