Good point, I guess the power company gives you A & B for free and no one cares about that heat content. We only care about C & D.
I think you're also right in that the insulation hardly notices the change in temperature. Once the initial energy is expended to get the tank up to the higher temp it wouldn't cost a noticeable amount to keep it there.
I guess this assumes that people use the same number of BTUs in every shower. But I suspect that if the water was warmer then the showers would get longer and consumption would rise, so maybe the overriding factor is human behavior. The colder the shower, the less energy that's used. Maybe I'll experiment with our teenager for the next couple weeks...
Our solar-heated water tank usually starts the day around 125 degrees and ends at 145-150. It has all the best insulation but the exterior of the shell is still warmer than ambient, and I'm sure it's heating up that cool, dark garage. (I even have piping insulation covering the hex nut to the anode rod.) Of course solar systems have a long recovery time (especially after sundown) so that game is all about getting the water as hot as possible while the sun is up and then enduring until the next sunrise. The best time of day to run the dishwasher and showers is around 1 PM...
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