It depends on many factors
Itís based on max temperature rise
So itís usually based on degrees above ambient.
So your water reservoir + x amount of degrees.
From what Iíve found, 20 years working with residential remodeling
Hereís proof (my user name .com)
The stated GPM is generally not accurate
For multiple reasons
1. These are based on house hold use cases. If Iím in a home I donít care if I use extra gallonage waiting for the water in the line to clear out, so the water is hot.
If I were using a 1.5gpm head and wanted to use a sink simultaneously I would get a 6gpm system
I donít know what others are recommending but i would recommend double whatever you plan on using in GPM if you want shore power like heating.
In my I plan on using
1 rain head (1.5 GPM)
2 shower heads (1.5gpm)
2 sinks (1gpm)
I want them to be able to run simultaneously
I plan on installing an 11gpm system.
This is DEFINITELY excessive
But the point in trump to make is that the rating they give are based on perfect lab settings. Better to upsize your system. The price difference between sizes itís negligible.