I installed solar panels and a 3 way valve so that I can divert a portion of the flow to the panels, or shut them off completely. The recomendation is to adjust the valve so that you get a 1 psi rise in pressure which should give the recommended flow rate through the panels. Even looking at the pressure loss vs flow cureves for the panels, 1-2 psi is where I wanted to be. So I adjusted the valve for a 1 psi pressure increase. I noticed the temp rise in the pool after a full days worth of sun and I was happy with the results.
However, I recently purchased a 3 way valve to install on my inlets and noticed that these valves significantly reduce the flow rate. If the gate is positioned for equal flow between two of the three ports, it blocks 2/3 of each port. Turning off the solar and then adjusting my outlet 3 way valve, I noticed a 2-3 pis increase just by turning the valve slightly. So, I adjusted my solar valve so that I had a 4psi increase, and now I am getting incredible performance from my solar panels.
Its dishartening to realize that the 3 way valves restrict the flow so much. On my outlets going to the spa and pool, I've tried positioning the valve such that all ports were open, but then my spa waterfall is pathetic, so I need the 3 way positioned in the middle.