Now for the kicker!

It sounds like you are planning on using this unit for a whole house water heater and I would consider it to be woefully inadequate in that application. In fact even most larger electric tankless water heaters yield disappointing results in whole home applications except in the extreme southern states where winter water supply temperatures are still quite high. At it’s best scenario hooked up to 240 vac power it will provide 3 gpm of hot water with the incoming water supply being 87 Degrees F! Now lets lower the incomming water temperature to 55 Degrees F and you will have only 1.5 gpm of 120 Degrees F water! 55 Degrees F would be a typical year round temperature from an underground water supply such as a well and in northern states municipal water supplies may be as low as in the 30’s which will make your hot water supply have a temperature right around 100 Degrees F. A shower will flow about 2.5 gpm, a sink about 2.2 gpm, washers and tubs will flow about 6-10 gpm. Is this starting to sound like you might not be happy with this unit? Even if it is just one person in this small home I doubt you will be satisfied.

Here is a link to some additional information that may help.
http://411plumb.com/tankless-water-heating-guide

If your heart is set on tankless I would recommend looking to a larger electric unit if you are in the extreme southern states or, if further north looking into a gas tankless unit which is able to supply larger volumes with a higher rate of rise. The gas unit would have to be vented to the outside.

The cheap way out is a tank type electric water heater and they are available in a variety of configurations ranging from short anf fat that can fit in a corner under a kitchen counter to tall and skinny in a closet.

I hope this helps you out in your quest for space saving and being happy with the volume of hot water you need.