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We have an off-grid customer whose PV arrays seem to be under
performing and we're not sure why. The customer has 16 Unisolar US64
modules, which have a peak output of 3.88 amps, but which were only
putting out from 2 to 2.4 amps each. He has a C40 charge controller,
so they were operating off the peak of the I-V curve (they were
operating at 13.6 volts each at the time of the system inspection,
instead of the peak, which is up around 17 volts), but still the amps
that they were putting out were lower than they should be. He used to
get 30+ amps (at 24 volts) from his array, but now he's only getting a
little more than 20 amps (611 watts from a 1024 watt array is what my
tech saw when he was there a week ago). The C40 was still in its bulk
cycle, not having reached the absorption set-point. The sunshine was
strong (there was no shading all day), there was no haze in the sky, it
was solar noon (1pm MDT), in the mid to high 70's with a breeze, the
arrays are on tilt-up racks, and there was no more than a little dust
on the modules. His arrays are at a year round setting (~latitude), so
not optimal for this time of year, but not that far off. Half of the
modules are 12 years old and half are 10 years old, but it was only
last year that he had 30+ amps coming in, so they couldn't have
deteriorated/aged that much in one year.<br>
<br>
Since there is no combiner box (we didn't design or install this
system), each pair (string) was checked with a clamp-on meter and was
putting out only 2 to 2.4 amps. When the C40 was bypassed, there was
no increase in current coming in, so the controller doesn't appear to
be at fault.<br>
<br>
The customer's measurement of the performance of his system is the LED
on the C40. He has a meter face on his C40, but he mostly just looks
at the LED. He says that the LED used to go solid green by noon, and
now he doesn't get more than two or three blinks all day.<br>
<br>
Any idea why all his modules might be under performing?<br>
<br>
Thanks<br>
Russell Mueller<br>
Positive Energy<br>
211-M Cruz Alta Rd.<br>
Taos, NM 87571<br>
575-737-9553<br>
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