two-speed with solar panels

[chem geek]

I don't know what kind of solar panels you have, but take a look at this link that shows the change in efficiency at different flow rates. It doesn't drop nearly as fast as your power consumption will drop at lower flow rates. The tradeoff will be somewhat slower heating of the pool with much larger savings in electric cost. Very roughly speaking, in a variable speed pump, the GPM is roughly proportional to the RPM while the power consumption varies as the cube of the RPM. You can see from the solar panel efficiency graph that going from 4 GPM at 80% efficiency to 2 GPM at 70% efficiency isn't a huge drop in efficiency, but that the power consumption will drop by a factor of 8 (a little less than that due to "fixed" losses). With a 2-speed pump, you may not be at the "sweet spot" of reasonable efficiency and low power consumption, but you just might.

Richard