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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