5 gal/min? My showerhead is 1.75 gal/min; and, yes, it has plenty of pressure.
Also, you don't need to be heating your water to 150 degrees (65 + 85). In fact, that's dangerously hot. Even if you did have your water heater set that high, you'd be diluting the stream substantially with cold water to get it back down to a comfortable temperature.
Between these two, it would appear that he's easily off by a factor of 5 or so.
I can't find that MacKay is "off by a factor of 5", as I can't see the that the calculation "wallflower" mentions is actually in the book. It seems to be "wallflowers" calculation.
However, this is what the book encourages. To learn how to make your own calculations and understand the numbers bandied about in the media about energy and energy systems.
Also, you don't need to be heating your water to 150 degrees (65 + 85). In fact, that's dangerously hot. Even if you did have your water heater set that high, you'd be diluting the stream substantially with cold water to get it back down to a comfortable temperature.
Between these two, it would appear that he's easily off by a factor of 5 or so.