NWS Heating Element

You can get a high wattage element from a hardware store and use it at lower voltages/currents:

http://www.homedepot.com/p/4500-Watt-240-Volt-Low-Watt-Density-Watt-Fold-Back-Screw-In-Stainless-Steel-Alloy-Water-Heater-Element-15011/204834493

• P=I^R=V^2/R=V*I
• R = V^2/P = 240v^2 / 4,500 watts = 12.8 Ohms
• P = V^2/R = 24^2 / 12.8 Ohms = 45 Watts
  
• P = V^2/R = 48^2 / 12.8 Ohms = 180 Watts
  

If you can find higher rated power elements, you can figure out the actual heating at lower voltage.

http://www.homedepot.com/s/water%20heater%20elements?NCNI-5

Also, lower rated voltage / wattage (AC input) does not mean lower power at lower DC voltages:

http://www.homedepot.com/p/1500-Watt-120-Volt-High-Watt-Density-Screw-In-Water-Heater-Element-15024/204834407

• P at voltage = element rating at AC power * DC volts ^ 2 / AC volts ^2 = 1,500 Watts * 48v^2 / 120v^2 = 240 Watts
  

One thing to watch for--The AC thermostats are not rated for full power at DC volts... It is very possible for a non-DC rated thermostat to burn out the contacts or even spot weld them together (short).

-Bill

I don't work for NAWS, and I rarely talk with them (just details about the forum).

I would guess that elements are Similar between vendors (and there are good and bad).

Low volume stuff (solar only stuff) is going to be more expensive.

If you have questions, please give our host a call. They have more information than I.

-Bill