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That was probably one of my formulas but the flow rate is fairly high for the RPM.
Running through my model I get about 107 GPM at 44 ft of head. 1500 RPM is half of that or about 53 GPM. Flow rate will always be proportional to RPM unless you have check valves, then it tends to drop faster with RPM. I assumed 35' runs and only one 2.5" suction line and one 2" return line plus typical equipment.
From my numbers for my IntelliFlo VF, with solar off I have 1500 RPM with around 25 GPM at 4-5 PSI (9-12 feet) while at I have 3000 RPM with around 70 GPM at 18 PSI (42 feet), but my pressures do not include suction head and I have somewhat long pipe runs (2" pressure-side; two 1.5" suction side).
I do realize that pump can operate at any speed. However the thing I would be concerned about is the electronic complexity of the thing. A two speed motor is just another set of windings and a switch but when you get into continuously variable speed motors you have a lot of electronics involved and it operates in a relatively harsh environment to boot. Granted the reliability of the electronics has gotten very good, especially in the automotive underhood environment which is extremely harsh from a thermal standpoint. However, the pool environment is harsh in some ways too, namely power line spikes and lightening. I also realize there are lots of electronics in the home today that survive spikes quite well. Just my comments. I believe simpler is better. I think once you get to know your pool you can settle in on runtimes and speeds that will work no matter what the math says.
Al
Personally, if one can afford the IntelliFlo VF pump, I'd go for it. I love mine and there is no way that I could have tuned my system as well as I did with a standard 2-speed pump. The savings I get when the solar system is on by running at 48 GPM instead of the 55 GPM with my previous fixed-speed pump is considerable while the savings I get with the solar off running at 26 GPM with the IntelliFlo compared to a 2-speed pump that might have been at around the same speed is also considerable.
If one cannot afford a variable speed/flow pump, then a 2-speed pump is still much better than single speed and will still save a lot in energy costs -- just not as much savings.
All true except that you really need to look at the lifetime costs of each solution and not just the energy savings. While the Intelliflo does better for energy usage, the cost premium, interface panels and/or controllers can make the 10 year cost higher for the Intelliflo than a 2 speed. So it really depends on the situation. Of course if you are only worried about carbon credits, then the Intelliflo is certainly the most ecological solution. :D