This may be a small tangent, but, another point to consider is the use of a diversion load from a solar/wind charge controller. This diversion load circuit can include a 12v heating element installed in the hot water tank in place of the 110v element. Once the charge controller satisfies the battery banks, it diverts usable voltage to the diversion load. This is something I am considering but have not yet tried or even fully investigated.
I've installed this diversionary system and it does work, but is dependent on the ability of the charge controller. The Outback has a very reasonable interface that's user friendly and easy to hook up. The element issue is neither here nor there as an element is merely a resistor and doesn't care about voltage (it's about cycling and amps). A small 2 gallon electric (110 or 220V) hot water tank (from your local home hardware store) with the diverted load wired through the thermostat works just fine. The only problem is when you're creating a large amount of diverted heat; you'll need a cold water mixing valve to prevent scalding.