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On a couple of Enphase systems I monitor regularly, the modules are
oversized compared to the inverters. One system has Sharp 224 W
modules and M-190s (capable of producing 199 W). The array is a
ground mount at a 30 degree tilt at 39+ degrees latitude. <br><br>
When the system was new in the autumn, I noticed two days of some
clipping. Since then, none. In the summer, when the sun angle
is better, the heat will keep the wattage down so clipping will very
seldom occur. The modules will not be close to STC, so the
inverters should not be pushed to their maximum.<br><br>
The same is true of string inverters. Since real power of the
modules in generally significantly below the nameplate, inverters will
generally not run at maximum power. When we take in all the
factors affecting the actual array power, I think that 120% over sizing
from nameplate wattage is generally safe. <br><br>
Drake<br><br>
<br>
At 09:14 PM 1/19/2011, you wrote:<br>
<blockquote type=cite class=cite cite="">David,<br>
<br>
Thanks for sharing that paper. The labeling the graphs for the
10-second and 1-minute data in Freiburg appears to be reversed - the one
minute averaging seems to have more data in all of the bins above 1000
W/sq m. Basically, these graphs show that irradiance observations
above 1100 watts per square meter are fleeting and disappear in hourly
averages. Such occurrences are also masked to a small extent
by 1-minute averages.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br><br>
<br><br>
David Brearley wrote: <br>
<blockquote type=cite class=cite cite=""><font face="Calibri">This
reminds me of a scholarly article I came across about a year ago while
doing some research. Here’s a link to it if anyone is
interested:<br><br>
<a href="http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf">
www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf</a>
<br><br>
David Brearley, Senior Technical Editor<br>
<i>SolarPro</i> magazine <br>
NABCEP Certified PV Installer ™<br>
<a href="david.brearley@solarprofessional.htm">
david.brearley@solarprofessional.com</a><br>
Direct: 541.261.6545<br><br>
On 1/19/11 12:29 PM, "Bill Brooks"
<<a href="billbrooks7@yahoo.htm">billbrooks7@yahoo.com</a>>
wrote:<br><br>
</font>
<dl>
<dd><font face="Calibri" color="#1F497D">Kent,<br>
<dd> <br>
<dd>How often were your data records? To capture edge of cloud effects,
you need one-second data. Not many people gather that fast or that much
data on inverters. I don’t think there is that much energy in these
spikes, but they are real and make some difference. 15-minute average
data will completely wash out this data.<br>
<dd> <br>
<dd>This is also a deficiency in modeling software since most models are
using hourly data.<br>
<dd> <br>
<dd>Bill.<br>
<dd> <br>
</font><br>
<dd><font face="Tahoma" size=2>From:</b>
<a href="re-wrenches-bounces@lists.re-wrenches.htm">
re-wrenches-bounces@lists.re-wrenches.org</a>
[<a href="mailto:re-wrenches-bounces@lists.re-wrenches.org" eudora="autourl">
mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] On Behalf Of
</b>Kent Osterberg<br>
<dd>Sent:</b> Tuesday, January 18, 2011 1:18 PM<br>
<dd>To:</b> Wrenches; Marco Mangelsdorf<br>
<dd>Subject:</b> [RE-wrenches] Inverter oversizing<br>
</font><font face="Times New Roman, Times"><br><br>
<dd>Attached is a graph that I produced to document the effect of various
ratios between the PV array size and the inverter size. I extracted
output power data for a 1020-watt system located in NE Oregon that is on
the Sunny Portal
<<a href="http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US">
http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US</a>
> . The data are publicly accessible so feel free to run your own
analysis. Better yet, analyze the data for a system near you.
<br><br>
<dd>Using 2009 data, I looked at how much energy would have been lost if
the output was clipped at 800W, 810W, .... 1020W. I used 2009 data
because there was a period in 2010 when the Sunny Webbox didn't have
internet access. At 800 watts, power clipping would have happened
on about 25% of the days. Yet the energy that would have been lost
was only 0.38% of the annual total. <br><br>
<dd>The results shown on this graph aren't universal, results would be a
little different in 2010, it would be different in some other climate, it
would have been different at another elevation, it would be different
with a different array angle, ..., and the module tolerance and inverter
efficiency also effect the results. Modules in this system are
Suntech 170-watt +/-3%. The inverter is Sunnyboy 1800 that
should be operating at close to 93% efficiency.<br><br>
<dd>Kent Osterberg<br>
<dd>Blue Mountain Solar. Inc.<br><br>
</font>
</dl><br>
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