Am I being thick?

[langstroth2]

4kwp array. 20 degree pitch. Two Strings. Data below taken on a clear sunny day, the system is under load/generating. My problem is I can't work out whether the numbers are as expected / there's an array issue as comparing with a nearby array it seems to produce less power / or I just stop worrying!

Here's a screen shot from Sunny Design:


and here's the panel spec:
Pmax: 250W
Vmp: 49.08V
Imp: 5.09A
Voc: 60.36V
Isc: 5.56A
Eff: 14.7%

If I ignore VoC and ISC as I understand they are not relevant under load; and use Imp and PMax and Vmp. I also understand that these are figures under STC - and obviously this isn't "real world" - so I'd expect to see real numbers a % below.

So:

STC 1 string (panels in series) in theory is:
8*250W=2000W
8*49.08V=392V
5.09A

Where's as actual reading for string 1 is:
1696W (/250W=6.78 panels)
357V (/392=7.27 panels)
4.745A (7% drop)

This equals 7 panels. I am really "losing" a panel (and a bit) per string due to de-rating/pitch?

 

[GaryM]

As you say, the figures on the panels are at Standard Test Conditions (STC), being a temperature of 25 degree's C, an irradiance of 1000 W/m2 and an air mass 1.5.
The figures you're seeing will likely be in conditions not equal to Standard Test Conditions so you should expect to see a difference.

 

[mdovey]

Your total output of the tow arrays is 3365W which seems pretty reasonable to me for a 4kW array in Spring.

 

[langstroth2]

Thanks for the replies. Perhaps I was being a bit obsessive (must get out more!) Some times comparing output with other nearby systems can be a double edged sword - you start wondering why one is "better" than the other.