There are a lot of benefits to the pumps. One is because they include the controller. All you do is get a 20-30w PV panel. I do a lot with these pumps and they have 3 sizes, the smallest being for 2-3 panels and the largest is big enough for 9-10 panels. It is no problem to have a diverting valve that switches over to a DHW tank.
Positive displacement means you can run 3/8" copper tube which is much less expensive as well as the insulation so there are saving there as well. Also, I have some of their pumps in 20+ year old systems.
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