Pmp temp coefficient math?

Yes panel temp has nothing to do with insolation. Mystery why they choose 800 to go with their NOCT (45C) list of specs. Topic raised in my thread on panel to controller loss near the end for 25/26 May.

here is my post on that:

Posted: 05/26/15 09:17am Link | Quote | Edit | Print | Notify Moderator

"How can you be a NOCT doing 1000 thingies?
And how are you measuring irradiance? "

It is normal for the panel temperature to be high when the panel is out in the sun. Usually about 20- 25C above ambient. So whatever the insolation amount is , the panel will be hot and so below its rated wattage.

The spac coefficient for that power loss lets you calculate that loss. Eg, the earlier mentioned 45C -25C = 20C difference and -o.43%/C times 20 is 8.6% and 8.6% of 255w is 22w, so panel wattage now at 255-22 = 233w

Meanwhile the panel spec for NOCT of 45C, also uses 800 form some reason. It could be anything including 1000, but they picked 800 and give the specs for 45C and 800, which are fairly low:

185W, Vmp 27.5, Imp 6.71, Voc 34.4, Isc 7.29

However, at the time I measured Voc at 35.1 (37.4 at STC), Isc at 9.12 (9.00 at STC) and panel at 45C. ( was about 20C ambient)

So my calculation, seeing 215w output (12.7v x 17a) and starting with that 233w panel, is that the missing 18w went to 4w for line loss as measured and that leaves 14w for controller loss, which came out as reasonable with 1.7% line loss and 94% controller efficiency.

I use the Isc as my indicator for being at or over 1000 insolation because it does not go down with panel temp but rises a little, so IMO 9.12 vs 9 says insolation was about 1000.

So that is the normal situation, with a hot panel and good insolation.

IMO listing the panel spec for NOCT with 800 is misleading. If they changed the N to mean "normal" instead of "nominal" (whatever "nominal" is supposed to mean !) then IMO they could list the likely specs for a panel temp of 45C and 1000 insolation, like I usually get. Actually I usually see 50C panel in July and August so panel wattage is even lower.

That would mean they have to admit that it is "normal" for the so-called 255w panel to be at 233w !

I'm an electrical engineer with two engineering master's degrees so maybe I can help, but I have no idea what "PMP", "(STC)", or "NOTC" means.

The temperature coefficient gives you the change in performance for a change in temperature. So if you get 101W @ 45 degrees C and the temperature coefficient is -0.46% per degree C, then a rise of 10 degrees C will cause the output to drop 4.6% (or retain 95.4% of performance) for a new performance of 96.54W @ 55 degrees C.