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This was a good read I found helpful - a May 2012 publication from the U.S. Department of Energy:
http://www.nrel.gov/docs/fy12osti/54242.pdf
It describes the energy savings of variable-speed pumps, and how best to operate them. I see now how important the skimmer is to my pool! It's much more than just getting leaves, grass blades and big bugs off the top of the pool's surface.
Here's a good quote: "For example, if you reduce the pump speed from 3,450 rpm to 2,400 rpm (30% reduction in speed) the wattage drops from 2,000 watts to 593 watts (70% reduction in power)."
And here's another, longer quote:
"A variable speed pool pump will allow the homeowner to achieve the ideal filtration flow rate with the least amount of energy consumption. Variable speed pumps utilize either Permanent Magnet Motors (PMM), which use permanent magnets to create a magnetic field between the rotor and the windings. This configuration is similar to the motors used in hybrid cars. Efficiencies are gained by the magnets working to spin the rotor, as opposed to a standard induction motor that requires additional electricity to induce the magnetic field into the rotor. The PMM motor design is much more energy efficient when compared to the standard induction motor, achieving efficiency ratings of 90% while the average single-speed pump will have efficiency ratings between 30% and 70%. PMM pumps can produce the same gpm flow rate as single-speed induction motors if needed; they simply run much more efficiently."
Another goodie! "A reduction in 450 rpm almost unnoticed in the application, cuts the power almost in half."