Quote Originally Posted by mas985 View Post
You can usually go by pressure rise. A 2-4 PSI rise is typical but it depends on the type of panels you have. If these are the kind of panels that have both the supply and return on the same side of the panels, they have a lot more head loss so a PSI rise of 5 may be more appropriate.

But the method that swimdaddy outlined will not give you a very accurate flow rate measurement. You are changing the plumbing which changes the head loss and flow rate.
Ya, that's what I thought might happen, thanks for clarifying!

Combining what Carl and you said, The panels seem to start feeling cool with a 2 PSI increase. I have a similar solar panel heater that is 4.5ft by 20ft with 4 loops all in series.