Boiling is turning water into steam / vapor.
It takes relatively little heat energy to raise the temperature of water to reach its boiling
point.
The BTU (British Thermal Unit) is the standard heat energy measurement. One BTU is the energy it takes to raise one pound of water one degree Fahrenheit.
A pint of water weighs approximately 1 pound.
So, to cook with a lb / pint of water, it requires 140 BTU's to bring the water from 72°F to 212°F - of water (this is important).
To keep the water at 212°F takes very little energy - just enough to match the heat lost to radiation, evaporation, convection and conduction. There are to many variables to estimate it.
It will take more BTUs to return the water to 212°F after we add food. Again, to many variables.
But, no matter how many BTUs the stove burner makes, the water will NOT get hotter than 212°F. All heat energy added to the water at that point is ‘used’ to change the water into steam, ie. boil it.
When all of the pound of water has been vaporized to steam, the stove will have added 970 BTU's to that pound of water to turn the 212°F water into 212°F steam.
That is almost 7 times the energy it took to heat the water from 72°F to 212°F!!
Now, I know we don’t boil all of the water in a pan to steam (at least not on purpose).
But.. it IS taking ‘extra’ BTUs to boil the water, and all it’s doing for us is showing us that the water is at 212°F.
A simmer, slow, soft, fast or hard boil just indicates how fast we are putting ‘extra’ heat-energy into the water. Remember, water never goes above 212°F.
My question: Would it be more frugal / efficient to use a thermometer instead of boiling the water?