[RE-wrenches] voltage drop from online data?

[Starlight Solar]

Hi Phil,

You mentioned several things to check. There are small losses at every  
connection, like combiner or disconnects, in addition to the actual  
wire loss. Something else to consider are the modules tolerance  
ratings. A module with +/- 9% like BP will probably have 9% less power  
than the nameplate rating as a starting point. Whereas a Sanyo with  
+10/-0 will most likely provide full power, all thing considered.  
Another thing you may want to measure is irradiance. If you have 800 w/ 
m2 striking the array, you will need to adjust for the difference. Oh,  
and don't forget to deduct for angle of incidence. Is the sun  
perpendicular to the module?  I probably don't need to mention age or  
soiling. Guess I just did.

Larry Crutcher

On Mar 17, 2009, at 10:02 AM, Phil Schneider wrote:

> Wrenches,
>
> I was looking at some of our web-based monitored systems this  
> morning to
> check in on some performance numbers, and this question came up.
>
> I can see a measured DC array voltage on-line (as measured at the  
> inverter),
> and I have cell temp information so I can calculate the theoretical
> temp-adjusted Vmp (based on module nameplate).  When I look at the
> difference between theoretical and measured, it seems that this can  
> largely
> be attributed to voltage drop in the DC wiring between array and  
> inverter.
> Am I missing other factors that would lead to this difference between
> theoretical and measured?  Power-point tracking errors,  Vmp  
> tolerance and
> module mis-match, cell temp accuracy are a couple that come to  
> mind.  I
> realize that field measurements at different points in the system  
> would give
> me more information from a scientific standpoint, but in this age of  
> remote
> monitoring I make sure we understand it's utility.
>
> Phil Schneider
>
>
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