Pump Sizing Question

[PoolDoc]

With a sand filter, optimum performance is to filter at 12 GPM per sft or less and backwash at 15 GPM per sft. So for a 30" filter, maximum (optimal) filter rate is (1.25^2) * 3.14 * 12GPM/sft = 60 GPM. Optimal backwash would be 72 GPM. So, that's one constraint.

If you select an 8 hour turnover rate for your ultimate pool, than a 60GPM system would match a pool up to 28,800 gallons. If you use a 24" sand filter, your filter SFT is (1^2 * 3.14) or 3.14 sft, and 38 GPM. That matches a pool up to 18,000 gallons. I wouldn't go smaller than that, if you want high quality water results.

With the solar unit, you'll need to find out if you have to install a suction relief valve are not. If you do, your pump & filter have to maintain a pressure of at least 10 psi at the filter discharge, at all times. Without a suction relief, you can start on high, but then switch to low once the lines are filled with air.

Given the changes you're planning, I think you may find it hard to calculate all your flow losses.

I'm not really in love with the electronically complex multispeed pumps like the Pentair Intelliflo (link), but I think in your circumstances, that might be the most practical and effective choice. Installing a few gauges and a flow meter, like this one, Blue White 2" flow meter - (Manufacturer info) , would make it easier to adjust flows once everything is in place.