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Posted by stevey on July 25, 2010, 7:53 am

Generally, according to NREL-Colorado the nominal performance when
new, within first two years, 77% is good.   However, dust is ever
and usually account for 5% of loss; unless you get rain often, and
a good tilt angle, or that you wash it down often.
May the Sun always warm your face and your panels!

Posted by Martin Riddle on July 25, 2010, 3:05 am

Include Panel dust too.

I believe the efficiency turns out to be somwhere around 75% of the
advertised panel power.
72% if you want to be exact for a grid tie.

I think someone here , with the same question, was seeing values in this


Posted by Ron Rosenfeld on July 25, 2010, 1:40 pm
 On Sat, 24 Jul 2010 23:05:27 -0400, "Martin Riddle"

I wonder if the "advertised panel power" is using STC or PTC, and
whether the calculations first compensated for temperature.

For design purposes, I usually use 80% of PTC to account for aging,
and I do use temperature compensation.

I haven't added anything for dust, but my panels, when there's no snow
or frost on them, seem to put out at least their rated STC power from
time to time.  

Unfortunately, I have no way of measuring independently what the sun
is doing at the time, so it's certainly possible that there is more
than the assumed kW/M^2 falling on the panels.

From an NREL database, the highest incident solar at my site is 1.22
kW/M^2, and values over 1 only occur for 138 hrs/yr; but that's based
on a 20-30 yr average.

Posted by Martin Riddle on July 25, 2010, 6:25 pm

Panel dust and atmospheric dust (eg dirt, smog) has an effect. Depends
upon location as to the amount. I have some info somewhere, if I
remember correctly its in the 3% range.

Something associated with Global Dimming ;)  1Kwm some think is
unrealistic. And conservativly use 800wm instead.

I have some info somewhere, I'll try to dig it up tonight, got something
to do before this heavy wall of rain hits NY.


Posted by Martin Riddle on July 26, 2010, 3:05 am

I have notes that indicate 900wm^2  is a realistic value to use in
Inverter effeciency 94%
Wiring loss 2%
Dust and wire connections 10% loss.

.9 x .94 x. 98 x .9  =  74.6%  system efficiency.


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