I installed a solar array for a client about five years ago that had a fairly large all electric house, including the biggest electric tankless I have ever seen. I belive it was made by Bosch. The thing that caught my immediate attention was that it required two 30 amp and one 40 amp 240 volt breakers. The client explained however that he was very happy with the setup becuase most of the time it drew zero power, but if the kids and grandkids were all in residence for the holidays he never ran out of hot water. A unique situation to be sure but a good solution.
I've installed gas tankless heaters in two of my previous homes and for the most part were pleased with them. One however would act up occasionally because the well water occasionally had a small amount of very fine silt the would get by the inlet filter. After I figured it out it would onlky take a few minutes to remove the water metering valve from the front of the unit and rinse it off.
In my current home I installed a Rheem Marathon tank water heater and I'm so pleased with it that I will probably install another in my next house. The tank is one "tupperware" plastic hull (like used in kayaks) moulded inside another and then they blow 3" of foam between the two hulls. Its efficient, has a lifetime warranty, and is very light and was easy to install. Perhaps the smallest version of the Marathon series or an electric tankless would meet your needs?
The garage space were I placed my 70 gallon Marathon has 12' walls and I was able to weld up a rack that elevated the water heater by four feet and placed this next to my basement woodstove. Then I formed 60' of 3/4 soft copper pipe around the woodstove's flue pipe and tee'd the top hot outlet from the loop into the Marathon's outlet to the domestic water and radiaint floors, and tee'd the return from the water heater drain back to the feed to the coil on the flue. It works simply and perfectly using the convective rise all winter, and the natural striation of hot to cold water keeps the lower portion of the loop from pulling heat out of the water heater in the summer. My water heating costs in the summer are minimal, and in the winter are virtually nil. Two caveats to this system; one would want to install a mixing valve in the domestic feed because the water in the tank can get very hot, and second there should be absolutely no shutoff valves anywhere in the convection coil loop. As long the loop is open the water will flow constantly, but it there were a shutoff and the flow stopped it could result in a steam explosion. I did install a 180 degree boiler pop off valve in my loop and plumbed the drain down to a pail on the concrete floor just in case, but it only activated once during an extremely cold windy night and even then only relieved itself of a gallon of water.
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