Solar Illuminace vs Panel Output

I guess illuminance is simply reducing the voltage.

CA Traveler wrote:
How does solar illuminace effect panel output? From yesterdays observation at noonish:

750W, poly, flat, 100F ambient, Vmp=91.7V, Imp=8.18

Logged Varray=75V, Iarray=8.5A, Controller out=605W

Loads were greater than panel output so the controller was in max MPPT state. So it appears that the illuminace is keeping the voltage down but not restricting the amps???

BTW It was very hot all day so at 100F ambient those black panels had to rather hot. So why is the amp output greater than Imp? This raises a red flag for me about the discussions of significant reduction in amps when the panels are hot and PWM is better than MPPT? Wonder what my amps would have been at Jim's 11,000' playground?

No I'm not in denial about reduced amps at higher temperatures. But now wondering about all of the generalized comments about how much better PWM is vs MPPT when the panels are hot. Now wondering if this might be less general and apply to certain panels and/or controllers? There is nothing like having logged controller data to raise red flags.

You have to separate the components of voltage and amps with panel heating. Voltage goes down with more heat but amps go up a bit. Power goes down because the voltage goes down more than the amps go up, and also because there are more volts than amps in the watts.

What you get with a hot panel is reduced input power to the MPPT controller (which has a buck converter in it) which is then reduced further by the controller(which might also be getting warm at 100F (wow!)), leaving whatever output power to go to the batteries. Divide that output power by battery voltage and that's your charging amps at the moment.

Meanwhile the PWM controller takes the panel amps and passes them to the battery and doesn't care about the power being less in the heat as long as there is enough voltage overhead to charge the battery.

So you could get to a point where the output amps from the MPPT is the same or even less than what you would get with straight PWM from the same rated wattage of panels. But then you also run into the overhead issue when doing PWM 24-24 with 60 cell instead of 72 cell panels.

I was only able to compare 24-12 MPPT vs 24-24 MPPT with a 60 cell panel and that vs 24-24 PWM. So I did not get to compare 24-12 MPPT vs PWM with 72 cell panel vs 36 cell panel.

I posted results of testing all that a few times lately, if you search. I got less with 24-12 MPPT and the same but more with 24-24 MPPT and PWM with a hot panel (but not as hot as the OP's must have been)

here we ago again .. beat me too it.. its the HEAT reducing voltage

the HEAT reduces the output power , and from my observation with the panels i removed and tested, its the voltage that takes the greatest hit
and with MPPT lost voltage means Lost Conversion amps/watts

example uni-solar listed VOC 23v ISC 2.94 amps
hot ambient temp 100F VOC 20.49v ISC 2.91 amps
same panel tested dead-on with ratings when cool

solar radiance can be the same two days in a row, but if one day was an ocean breeze and the next was a HOT Santa Ana wind, the hot day is going to have a lower output

Sure but what does Voc have to do with the MPPT operation which uses Vmp?

Another panel spec is Isc and what does this have to do with a MPPT controller that uses Imp?

It’s just not apparent how you can use panel specs and temp coefficients to get a reasonable idea of what to expect with MPPT. Perhaps the bottom line is the controller output. For my flat panels at noonish I was getting 44A and 605W from my flat 750W panels at 100F ambient and perhaps 131F panel cell temps.

OK, heat reduces the output but by how much? At 131F my panel specs say I should get 658W and I got 605W, so perhaps 53W was loss due to flat mounted? But my panels are mounted 6” off of the roof so perhaps the panel cell temp is below 131F which is just a guess anyway.

We all use generalized panel information which is what we’ve got and what I used as a basis for my system. But having a quality controller that logs data under live operating conditions is proving enlightening.

Perhaps hotter. Add 30C to 100F and get 70C

70-25 = 45 ... 45 * -.45%/C = ~20%
750 * (1-.2) = 600w

"Sure but what does Voc have to do with the MPPT operation which uses Vmp?"

It doesn't "use" THE Vmp, it picks a Vmp from available V, which is reduced by heat.

It gets complicated, but reading a bunch of stuff on Google plus the owner's manual's for the panels and controllers gradually gets through till you almost think you understand it! :)

Taking measurements like the OP is the way to go, and using your basic math to work out where the watts and amps all got to works with the bumph you read to help understand what they are trying to say.

CA Traveler wrote:


OK, heat reduces the output but by how much? At 131F my panel specs say I should get 658W and I got 605W, so perhaps 53W was loss due to flat mounted? But my panels are mounted 6” off of the roof so perhaps the panel cell temp is below 131F which is just a guess anyway.

We all use generalized panel information which is what we’ve got and what I used as a basis for my system. But having a quality controller that logs data under live operating conditions is proving enlightening.

you lost more than you expected..
that is what you want to explain..

it is heat loss, IF you can.. try measuring the temp at the panel surface

PS..what i posted was just an example
if there is loss with a NO load dead short condition
extrapolate from there.. the loss factor when flat mounted, charging and hot as HECK