No idea who often claims that.
It is a fact that panel heating lowers the panel voltage so that typically, you lose 10% of the power because voltage which drops with heat is such a big component of watts compared with amps which go up a bit with heat. So you lose watts for using MPPT which has a buck converter that uses watts in and watts out.
Meanwhile PWM gets the prevailing panel amps to the battery up to and sometimes over the Isc value no matter what the panel heating is.
So for instance with my three 100w panels aimed at high sun each panel rated 6.3 Isc, I got 18.6 amps to the battery with PWM as seen on the Trimetric.
Nostalgic photo--those three are now on top of the Class C in sig.
Swapped out the PWM real quick, no change in the sky, for the MPPT controller set to 12-12, no change in wiring and Ta Da! 18.x amps not even 19. So same amps.
I did all kinds of measurements of temperature with my IR gun and tried different things with different panels and measured it all, did all kinds of math with the temp coefficients . I reported a lot of that on here back then. Most of my notes from then are now gone.
Anyway despite those studies that showed (one was Dutch ISTR) that what I was seeing was impossible, I got what I got. Everyone is invited to go out there and do the same thing, swapping controllers, etc, and see what you get.
I have now got a mix of MPPT and PWM controllers in parallel set to the same voltage on my roof arrays and they all behave adding their amps to the right amounts (as expected by me) so I do not have a dog in this race anymore.
I will stick with what I saw with my own eyes for real, never mind what I am supposed to see according to the MPPT enthusiasts, Dutch or not :) .
Note--I do not have the controller efficiencies for my PWM and MPPTs. the wiring was good for 12v and both controllers were 12-12. Panel temps taken on the back of the tilted panels aimed at the sun showed variations in temp with location on the panel--not the same everywhere, but more to where the sun was and up higher too. So panel temp must be taken as some sort of average and the voltage of each cell is added to get panel voltage remember. Anyway, it was all good fun playing scientist and seeing what was really going on.
