the correct way to calculate solar panels the 4x12'
Is as follows, each one needs 4 gmp flow for max efficancy.
so if you have 7 panels you should have a flow rate exiting the roof as 28 gpm.
Call it 30.
There is only one way to know that it not by the % of power or hp or anything other than simple flow rate meter.
this is done by a permanent flow gauge inline on the pipe.
I have been using these for over 20 years on aquariums and ponds