Storage tank geysers, like the ones we've been using to burst and flood our wardrobes for the last 100 years are sized in litres of storage capacity, such that a 150L geyser stores and (keeps warm) 150L at any time. When you turn on the hot water tap there is 150L pre-warmed water at your disposal. When you (or more likely your kids) have used up you 150L, sorry for you. You're out the game.
Instantaneous geysers have 0L storage capacity, and even the best marketing shyster is going to struggle to market that effectively. So we need another way.
We specify tank-less water heaters by measuring the amount of water it can heat up instantaneously in litres per minute. To make sure the guys on the far east of the map don't take advantage of us consumers, we also specify that the unit has to heat the water by a minimum of 25 ℃. So what we end up with is a measure inlitres per minute at Δ 25 ℃.
Single units range in size from 6Lpm to 32Lpm. In some cases units can be cascaded together to deliver hundreds of litres instantaneously.
So what size do you need to supply your home? You're asking the wrong question. The clever way to size your requirements is to askWhat's the total (hot water) taps that'll be opened at the same time?And be realistic, sure there was that one time when your doting wife and the kids and your doting wife's loving mother in-law all needed to shower at the same time, but ask yourself, if number 3 had a cold shower half way through would that really be a train smash?
In most homes, it's pretty rare for more than 2 hot water faucets to be opened at the same time.
According to thethe engineers, you can expect to use 19Lpm at your bath point, and 19Lpm at your shower point?!? And 17Lpm at the sink faucet. These are engineers though so you can bet that'll be calculated at peak. Also, those incoming pressures seem a little high.
The bottom line is, a 16Lpm will happily supply a single faucet. If you want a single unit to supply the whole house, then a 26Lpm fan assisted unit will do the job.