I'd really like to understand how these motors work in general ...

So a standard motor (under reasonable conditions, i.e. while hooked up to pool) will always run at maximum power and constanly draw the rated amps ? Even if the mechanical resistance of the pump varies due to e.g. a more or less clogged filter the amp draw is the same ? I am not questioning it, just want to understand the basics ... This would mean that the pump "tries to get the maximum flow it can achieve at the given head" and consumes always a constant amount of energy per unit of time.

I am trying to calculate cost saving / ROI for various replacement pumps and want to model actual consumption.

Regarding sizing the pressure gauge at the filter shows 15 psi (35 ft of head), so if I add suction side head of 8 ft I am at 43 ft of head - does that make sense ? The pipes around the filter and in the ground are all 1 1/2 inch PVC so I should not go higher that 44GPM which is fine for the 8-10 hour turnover.

Pump options are (some might be a little crazy)

- a regular EE pump, .5 HP such as the Northstar NS4005 although flow at 40 ft of head would be a tad too high for the plumbing

- a two-speed pump such as the smaller Pentair WhisperFlo WFDS-3
very tempting - a 115V - means I don't have to run new cables.

- PMSM based pumps such as Pentair IntelliFlo 100, 160 or the big one. They are expensive but might work out IF the energy savings are close to what Pentair claims. Do you have any idea how to estimate this ?

Thanks

Patrick