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Declining power from PVs?

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Posted by Scott Loupin on July 1, 2005, 1:28 pm
I just had a solar system installed last Fall.  THis Spring, on bright,
sunny days, we'd store up to 7kWh/day.

Now we're only getting 4.4 to 5 kWh/day.

The panels aren't shaded, so why would the power output drop?  How can I
test the panels?

Thanks for you help.

Posted by William P. N. Smith on July 1, 2005, 1:45 pm

Could be dirt buildup, so you could try cleaning them.  Physical
inspection would be important, to detect broken wires, cracked glass,
etc.  You can test them by measuring open circuit voltage and short
circuit current on individual (disconnnected) panels on a sunny day
and see if there's any difference between them.

Posted by Scott Loupin on July 1, 2005, 2:43 pm
 Hi William, thanks for the ideas.  They get rain-washed often, but it might
not take much dirt to cut back power....

No cracks, and the wires aboveground look OK.  I did have a dumptruck drive
over the buried wires, but that was last Fall.

How would I measure short circuit current?  Use a shunt block, or a clamp


<William P. N. Smith> wrote in message

Posted by R.H. Allen on July 1, 2005, 3:30 pm
 Scott Loupin wrote:

It takes a lot of dirt to cut back power. I've never heard of anybody
losing more than a few percent to dirt except in some pretty extreme
situations (sandstorms and the like).

What type of panels do you have? Amorphous silicon panels tend to lose
efficiency very quickly over their first month or so in the sun, then
stabilize at that lower efficiency.

Simply disconnect the panel from your system and short it through an
ammeter. You won't hurt anything -- really -- as solar panels are
self-limiting. Or more accurately, they're limited by the amount of
sunlight shining on them. Just make sure you have a big enough meter to
handle the rated short-circuit current printed on the back of your module.

Posted by daestrom on July 1, 2005, 5:03 pm

Well, since nobody else asked, "How are they oriented?"  What angles (tilt
and azimuth) and what latitude?

Could it be in the spring/fall they get a better angle to the midday sun
than now near the summer solstice?  If the sun is higher in the sky, at
noon, you could be losing some to reflection off the unit.  With fixed
mounts, the angle chosen is a bit of compromise between the seasons.


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