I had a look to see what info was out there on scales energy costs and found this pdf file
https://www.cwt-international.com/cw...Efficiency.pdf
Highlights below
Reduced efficiency
Over time scale deposits act as an insulator,
causing the water heater to work harder and
harder to heat the water.
Scale
reduces
energy efficiency of the water
heater by up to 50%
-
Each five grains per gallon of water hardness
will cause a 4% loss in efficiency
-
Water temperature decreases 5°C with a
limescale thickness of 2mm after 480 seconds
- A water heater‘s
useful life can be
reduced by
as much as
50% through scale build-up
-
Scale deposits will cause a 4% increase in cost
for gas storage tank water heaters when using
50 gallons of hot water per day.
-
0.5 mm of hard scale increases fuel costs by
9.4%
I have seen well water make a kettle 1/2 inch thick in calcium deposits and they do burn out water heaters there was one in the yard last time I visited
