I have moved this to the China Shop.
My low speed setting runs around 1/4 hp vs the 1hp full speed. My solar panels cannot be opened more than a crack at full speed because they will distort and leak, but at low can be opened all the way.

There is no possible way running at low speed for 24 hours will NOT use double the KWH of running at low speed for 12 hours.

Personally, I see Mark's point as being what is NEEDED to distribute chems evenly and actually filter the water. It doesn't take into account ancillary needs like skimming the surface, aerating to raise pH without raising TA, sufficient run time for an SWCG, or solar panels. Nor does he claim it does. He merely claims that a full daily turnover of water isn't really necessary.

As for cost savings, usually 2speed pumps can run far longer and more efficiently. Even if we go back to the touchstone of one turnover per day, what is the point of a 24/7 run unless your pump is 'way too small for your pool? If I can turn over my water at low in 10-12 hours, why run it doubly long? If I can turn it over in 6 hours at high, I will still use double the KWH of a turnover at low. You cannot magically squeeze more electricity out of a pump by running it twice as long AT THE SAME SPEED!

TA is a buffer for pH. I cannot explain the full chemistry of Carbonic Acid and Bicarbonate ions, as I leave that to my son, the Chem major and Chem_Geek. But generally, if pH tends to trend up, keeping TA lower can slow or arrest that. Likewise if pH tends to trend down, higher TA also controls that. And, if you have a vinyl pool and your pH is stable, TA is only important if it approaches 200ppm with a CH of 400ppm or more, because you can get cloudy water and scaling.

Since I rebuilt my pool my TA has been trending high, around 170 (local water). pH has been trending down, but that's been due to two floaters filled with Tri-Chlor tabs, which are acidic, but keep FC up and add CYA. Been compensating with low doses of Borax, so pH has stayed at 7.3-7.4. But in the past, even last season, TA was at 50-60 with no problems.