Originally Posted by s/v Jedi
You say that there is a 2% variation in efficiency among panels on just temperature correction? I did not know that but would love to see this supported by figures from manufacturers or test labs
Jedi - I'm posting
this in response to your question, and know you know the details, but laying some of them out for others who might not.
It's the temperature coefficient of power production. Power output goes down as cell temperature goes up and panels are rated at STC, which are unrealistic operating conditions. So a 100W isn't really a 100W across the board. Here are some samples (of well known, respected, widely used brands). As my example I took 250W panels so it would be easy to make comparisons.
Sunpower X20-250-BLK -0.30%/C (Sunpower X20 Datasheet
Sunpower SPR-245NE-WHT-D -0.38%/C (Sunpower E19 Datasheet
(an older 245W model - couldn't find the actual datasheet for the 250)
Solarworld SW250 -0.45%/C (Solarworld SW250 Datasheet
KD250GX-LFB2 -0.46%/C (Kyocera KD250 Datasheet
Just in those three manufacturers there's a range of 0.16%/C. Given that panels generally operate about 20C above the STC rating temperature (as evidenced by the NOCT standard), that means a spread of 3.2% in panel outputs under real world conditions, all for the same nominal power rating.
Another way to look at this is the NOCT/PTC power ratings of the panels. These are ratings under more typical operating conditions (including 20% less irradiance for NOCT, so an automatic 20% hit to power rating).
Sunpower X20-250 = 188W (NOCT) / ???(PTC)
Sunpower SPR-250-BLK = ??? (NOCT) /231.7W (PTC)
Solarworld SW250 = 183.3W (NOCT) / 222.3W (PTC)
KD250 = 180W (NOCT) / 223.7 (PTC)
All of the above listed panels are "250W" panels, but if you dig around in datasheets (the information is not always easy to find, even at aggregating websites) you can see that using NOCT rating conditions the panels range from 180 to 188W. That's a 4.3% difference for panels of the same nominal rating. These are all values published by the manufacturers, reputable ones can back them up with independent test results, other maybe not.
One of the things to note in all of this is that NOCT is the measured normal operating cell temperature, that is, at 20C ambient and 800W/m2 irradiance, what is the steady-state operating temperature? If you look at the Sunpower panels it is 41-43C, while for the others it is 45-46C. So the Sunpower panels stay cooler, giving them a better power output. There are numerous factors; more efficient cells convert more sunlight to electricity and less to heat, and different panel designs can be more or less efficient in passive cooling
(thickness of silicon, glass, coatings, frame,.....).
Ain't it grand that an apple isn't quite really an apple?