I think what I don't like about NOTC specs is the suggestion that getting 80% is normal, when I would get full Isc aiming at a high sun as being normal. I got the idea they listed the NOTC numbers as some sort of salesman excuse thing. I am muddled in my thinking about all that. "Nominal" does not equal "Normal" I guess.
These guys use "Normal" for NOTC. Only they call it NMOT. they use "nominal" in the warranty statement. They give a graph for low light compared with STC performance
http://www.wegosolar.com/products.php?product=HSQ%252d310M%252dBLK-Hanwha-310-Watt-60Cell-Solar-Panel-Black-FrameEDIT--I see my Canadian Solar brand panel specs use "Nominal" for NOTC. It probably makes sense in the original Swahili ?
To me it is so easy. Whatever the conditions or time of day, aim the panel and see what Isc is. Connect up and see that is the same as the amps to the battery with PWM and your equipment is working right.
I found that Isc per panel wattage is about the same so I can estimate what a 255 watt panel ought to do if it were PWM, then see what it really gets. I use a 130w panel with 8.2 Isc as baseline.
255/130 x 8.2 = 16 amps So I thought I would see more than 16 with my MPPT. Not really. So after much poking around it turned out to be panel heating reducing input watts so output watts was lower and divide that by battery voltage and there you are. It did come out a little ahead in the morning with lower Isc at lower sun, but mid-day about the same.
Salvo posted his results back when showing his MPPT did much better than PWM/direct early in the day, then the results got close by high noon.
I had the thought the 20 amp MPPT controller would clip the amps so it would be stuck at 20, but it was doing 18.x with MPPT compared with 18.6 with PWM in that test with three 100s.
Heat coefficients for 12v panels are not the same as with 24v panels so that might throw things off a bit, but not enough to matter.
