On Thursday, September 18, 2014 12:20:48 AM UTC-4, sol wrote:
GPM tells how large the water pipe that goes through the water heater is, not the actually heating capacity. You have to pay attention to watts. An electric tankless water heater has perfect efficiency so there is a strict maths that tells you how hot it can make the water. What you need to know is how much water flow you are going to require, how hot is good enough for you and you incoming tap water temperature. This explains it much better:

http://toastyreviews.hubpages.com/hu...heater-reviews

If you decided to do tankless, you definitely need to get something bigger than you think you need. There is going to be some heat loss in the pipes etc. so you can never get a too powerful electric tankless water heater. That's why many people say tankless sucks. They get a small device and it doesn't heat enough.

And the flip side is that a big one takes so many amps that it may
require upgrading the panel/service. Factor that in with the much lower
cost of an electric tank type unit and the tankless doesn;t look so
good. Also, many places have time of day metering now. If you can run
the electric tank WH during off peak, at night, etc, it can save a lot
in operating costs.



I doubt Rheem RT9 will do any good even for a single person, unless you are in a really hot region. 15 watts is the minimum for tankless electric.

You left out the K in Kwatts. I can see smaller ones used for special
applications, like if you had just a sink somewhere in a building that's
rarely used. That could be a good application for tankless and a small
one would fit. But I agree that you need a decent size one to do even
one bathroom with shower. Whole house gets worse. If you want tankless,
gas tankless looks much better, but I'm not a fan of those either, for
well known reasons, high initial cost being the main one.