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<DIV><FONT size=2 face=Arial>PVWatts is a good general estimator when
you fine-tune the derate factor. For single crystal and multicrystalline
arrays, I generally use 0.65 for battery-based PV and inverter systems and 0.82
for batteryless inverter systems. PVWatts annual results are l5% low
for Unisolar arrays because PVWatts uses the crystalline silicon temperature
coefficient.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Even though PVWatts2 may seem more accurate,
it does not factor in unique local climate conditions like California
coastal morning and afternoon fog or inland persistent winter Tule fog.
However, NREL's climate data does include LA's "June gloom" see <A
href="http://en.wikipedia.org/wiki/June_Gloom">http://en.wikipedia.org/wiki/June_Gloom</A></FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>For flat roofs in snowy climates like the Unisolar
project in Rochester NY, I deduct 30% from PVWatt's November through March
estimates for a fairly accurate annual estimate. Powerlight (now SunPower)
also uses an additional 30% monthly derating for flat roofs in snowy
locations.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>PVWatts does not account for really dusty and dirty
air locations. Bill Brooks worked at PVUSA and is very knowledgeable about
power loss from soiling from agricultural dust in central California. Bill
also measured 35% power loss at the Long Beach CA harbor
waste-to-energy powerplant PV systems. That location and most LA county
freeways experience particulate pollution that not only reduces PV
production but causes permanent respiratory damage to children and shorten the
lives of elderly people, sort of like the canary in the mine.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Joel Davidson</FONT></DIV>
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style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=kelly@whidbeysunwind.com href="mailto:kelly@whidbeysunwind.com">Kelly
Keilwitz, Whidbey Sun & Wind</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A
title=re-wrenches@lists.re-wrenches.org
href="mailto:re-wrenches@lists.re-wrenches.org">RE-wrenches</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Thursday, June 10, 2010 7:57
AM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [RE-wrenches] Real world PV
production</DIV>
<DIV><BR></DIV>
<DIV>Thanks, Don,</DIV>
<DIV>Performance predictions such as from PV Watts VASTLY underestimate PV
production in our area. For example, PV Watts estimates 940 kWh/kW for a
4:12 pitch at 180˚, whereas such systems are actually producing up to 1300
kWh/kW (as measured by a renenue-grade production meter). We have to set the
derating factor to nearly 100% in order for the predictions to match.</DIV>
<DIV><BR></DIV>
<DIV>I assume that is partly due to coarse irradiance data (not accounting for
higher irradiance for our location in the rain shadow of the Olympic
Mountains), but also due to our clear, cool, windy summers, good natural
washing, and (perhaps) dispersed distribution of irradiance.</DIV>
<DIV><BR></DIV>
<DIV>I want to know if the opposite is true: Do performance predictions
OVERESTIMATE PV production in areas with historically high irradiance, but
significant soiling and temperature issues.</DIV>
<DIV><BR></DIV>
<DIV>Thanks,</DIV>
<DIV>-Kelly</DIV><BR>
<DIV apple-content-edited="true"><SPAN
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<DIV>
<DIV>Kelly Keilwitz, P.E.</DIV>
<DIV>Whidbey Sun & Wind</DIV>
<DIV>Renewable Energy Systems</DIV>
<DIV><A
href="mailto:kelly@whidbeysunwind.com">kelly@whidbeysunwind.com</A></DIV>
<DIV>360-678-7131</DIV>
<DIV><BR></DIV></DIV></DIV></SPAN></DIV>
<DIV>
<DIV>On Jun 9, 2010, at 8:59 AM, i2p wrote:</DIV><BR
class=Apple-interchange-newline>
<BLOCKQUOTE type="cite">
<DIV name="Mail Message Editor">
<DIV><SPAN class=Apple-style-span><BR></SPAN></DIV>
<DIV id=replyPrefaceElement>On Jun 9, 2010, at 8:22:12 AM, "Kelly Keilwitz,
Whidbey Sun & Wind" <<A
href="mailto:kelly@whidbeysunwind.com">kelly@whidbeysunwind.com</A>>
wrote:</DIV><BR>
<BLOCKQUOTE
style="BORDER-LEFT: blue 2px solid; PADDING-LEFT: 5px; COLOR: blue; MARGIN-LEFT: 5px"><SPAN
style="WIDOWS: 2; TEXT-TRANSFORM: none; TEXT-INDENT: 0px; BORDER-COLLAPSE: separate; FONT: medium Helvetica; WHITE-SPACE: normal; ORPHANS: 2; LETTER-SPACING: normal; COLOR: rgb(0,0,0); WORD-SPACING: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px"
class=Apple-style-span><SPAN style="FONT-SIZE: 11px; font-f: monospace"
class=Apple-style-span>Thanks, Joel<BR>How about PV systems away from the
coast, in a hotter, dustier<SPAN
class=Apple-converted-space> </SPAN><BR>location, like Bakersfield,
Fresno, Sacramento, Palm Springs,<SPAN
class=Apple-converted-space> </SPAN><BR>etc........?<BR><BR></SPAN></SPAN></BLOCKQUOTE>
<DIV><SPAN class=Apple-style-span>In central CA we do a little better. I
casually monitor several systems in this area and expect around 1500-1600
kwh/kw. per year.
<DIV><SPAN class=Apple-style-span><BR></SPAN></DIV>
<DIV><SPAN class=Apple-style-span>Don Loweburg</SPAN></DIV></SPAN></DIV>
<DIV id=u9CD372ACEC35420CBD4E99C9A7DEA98B
class=aol_ad_footer></DIV></DIV><BR></BLOCKQUOTE></DIV><BR>
<P>
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