[RE-wrenches] voltage drop from online data?

[Glenn Burt]

Great article on performance calculations in the previous issue of Solar
Pro, pp24.
I made a quick & dirty spreadsheet that we use to plug in measured values on
site to give us a ballpark during checkups.

-Glenn Burt

-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Phil
Schneider
Sent: Tuesday, March 17, 2009 1:02 PM
To: RE-wrenches
Subject: [RE-wrenches] voltage drop from online data?

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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