I'm finding some discussions and one scientific paper about installing panels in parallel vs in series and everyone seems to answer, "it depends". There's got to be a general rule-of-thumb for say Southern desert California. or Arizona - just based on experience, no calculus or vapor dome calculations, please. I know if I would install 100 panels, putting them all in series ,the last 20 or so wouldn't gain any additional heat and the backpressure and water volume losses would be huge. The other (all or nothing) option is all in parallel. There the pressure and volume loss problem are eliminated but the water wouldn't have time or distance to pick up much extra heat. So I know the real world answer is some combination of parallel series of panels. I'm installing 16 large panels (4x12): how many runs (panels in series) will add the most heat with the best volume return to the pool? I'm thinking 2 runs in series of 8 panels but maybe 4 runs of 4 panels each would be best. You'd think the dealer or the manufacturer would have these answer but you'd be WRONG.