When it comes to getting the tankless hot water right in the home, tankless hot water heaters sizing is the key issue. If you are getting ready to switch from a tank system the ratings may be a little confusion - it is not like a tank where you can look how big it is and visualize how much water is in there! On the other hand it is not so hard either.
Any kind of the on demand water heaters will produce water for a long time - the point is that it does not run out. So it makes no sense to say it has an 80 gallon capacity like you would for your tank. Instead manufacturers of these units talk in gallons per minute. Once you have established this figure, the heater will go on producing this hot water pretty much forever.
So this is the first thing to sort out when getting the size right. There is no need to work out what all your appliances use, even if you use them one after another (this being the classic running out of water problem for tanks). Rather all you need to work out is the maximum demand. So think about times when you use water appliances simultaneously and add them up.
The remaining sizing issues for the tankless gas water heater is the incoming temperature of the water. If you look carefully you will see that the gallon per minute ratings are dependent upon the temperature rise you need to achieve. A simple call to the supply company should be able to tell you the answer to the average seasonal temperatures of your water supply. And then it is just a question of buying the unit you need!