<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Ray,<div><br></div><div>I agree with what you have calculated, but the temp. coef. listed is for a Voc. For voltage sag, the temp. value would be a Vmp value and I don't know how much of a difference that makes. I have seen some manufacturers offer a temp. coefficient for Vmp as well as Voc. Not sure what the difference would be, but if it gets down to a few volts with the PE, it is would be worth checking. </div><div><br><div>
<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; 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; font-size: medium; "><div><div><div style="font-size: 12px; ">Doug Wells</div><div style="font-size: 12px; ">The Solar Specialists</div><div style="font-size: 12px; ">Morrisville, VT 05661</div><div style="font-size: 12px; ">(p) 802-223-7014</div><div style="font-size: 12px; ">(c) 802-498-5856</div><div style="font-size: 12px; "><a href="http://www.thesolarspecialists.com">www.thesolarspecialists.com</a></div></div></div></span>
</div>
<br><div><div>On Nov 29, 2011, at 5:55 PM, Ray Walters wrote:</div><br class="Apple-interchange-newline">
<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type">
<div bgcolor="#FFFFFF" text="#000000">
I'm prepping for a class on this very subject tonight, so I'll just
wrap up everybody's good points:<br>
V min = Vmpp x (1- temp diff x Temp coefficient)<br>
<br>
So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean
Maximum, then add Scott's adder (ie roof mount off 6" = +30C) so
let's say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp<br>
then, 65C - 25C(STC) = a differential of 40 deg C from standard test
conditions<br>
<br>
The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for
numerous modules, <br>
Solar world for example is -0.33 %/degC<br>
<br>
With a 40 degC differential then, the voltage would change by 40 x
.33, or 13.2%<br>
<br>
So just assuming Vmpp = 35v, the adjustment would be 35v x
(100%-13.2%), or 86.8% of 35v, <br>
which gives a Volt min of 30.4 v<br>
<br>
Somebody please correct my math if I got that wrong.<br>
Also probably more interesting is the actual Power coefficient,
which shows the total power losses from higher temperatures, not
just voltage. The calculation would be similar.<br>
<br>
Ray<br>
<br>
On 11/29/2011 2:07 PM, Darryl Thayer wrote:
<blockquote cite="mid:1322600875.70711.YahooMailNeo@web39313.mail.mud.yahoo.com" type="cite">
<div style="color:#000; background-color:#fff; font-family:arial,
helvetica, sans-serif;font-size:12pt">
<div><span>I am on road so I have no texts with me, but the same
formula applies about 0.4% per degree (less voltage) or use
the module label voltage coefficent. However you must use
the MPPT voltage and the temperature = (temperature ambient
(2% high) plus roof temperature warming) The roof
temperature is hard to find good values because it depends
upon spacing between roof and module. I add 15 deg C for
most situations. </span></div>
<div><span></span> </div>
<div><span>Best I can do from memory.</span></div>
<div><span>DT</span></div>
<div><br>
</div>
<div style="font-family: arial, helvetica, sans-serif;
font-size: 12pt;">
<div style="font-family: times new roman, new york, times,
serif; font-size: 12pt;"> <font face="Arial" size="2"> <b><span style="font-weight: bold;">From:</span></b> Kirk
Herander <a class="moz-txt-link-rfc2396E" href="mailto:kirk@vtsolar.com"><kirk@vtsolar.com></a><br>
<b><span style="font-weight: bold;">To:</span></b>
RE-wrenches <a class="moz-txt-link-rfc2396E" href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a> <br>
<b><span style="font-weight: bold;">Sent:</span></b>
Tuesday, November 29, 2011 1:48 PM<br>
<b><span style="font-weight: bold;">Subject:</span></b>
[RE-wrenches] calculating low string voltage<br>
</font> <br>
<div id="yiv1276106914">
<style><!--
#yiv1276106914
_filtered #yiv1276106914 {font-family:Calibri;panose-1:2 15 5 2 2 2 4 3 2 4;}
#yiv1276106914
#yiv1276106914 p.yiv1276106914MsoNormal, #yiv1276106914 li.yiv1276106914MsoNormal, #yiv1276106914 div.yiv1276106914MsoNormal
{margin:0in;margin-bottom:.0001pt;font-size:11.0pt;font-family:"sans-serif";}
#yiv1276106914 a:link, #yiv1276106914 span.yiv1276106914MsoHyperlink
{color:blue;text-decoration:underline;}
#yiv1276106914 a:visited, #yiv1276106914 span.yiv1276106914MsoHyperlinkFollowed
{color:purple;text-decoration:underline;}
#yiv1276106914 span.yiv1276106914EmailStyle17
{font-family:"sans-serif";color:windowtext;}
#yiv1276106914 .yiv1276106914MsoChpDefault
{font-family:"sans-serif";}
_filtered #yiv1276106914 {margin:1.0in 1.0in 1.0in 1.0in;}
#yiv1276106914 div.yiv1276106914WordSection1
{}
--></style>
<div>
<div class="yiv1276106914WordSection1">
<div class="yiv1276106914MsoNormal">I am in debate
with a PE over calculation of low voltage of a
series string on a hot day. He insists that an
arbitrary high cell temp is factored in, not just
ambient temperature. Could someone please give an
accepted formula for this calculation? Thanks. I
cannot find a clear reference to low voltage
calculation on a hot day (but every reference
material is clear on how to calculate high voltage
on a cold day).</div>
<div class="yiv1276106914MsoNormal"> </div>
<div class="yiv1276106914MsoNormal">Kirk Herander</div>
<div class="yiv1276106914MsoNormal">VT Solar, LLC</div>
<div class="yiv1276106914MsoNormal">dba Vermont Solar
Engineering</div>
<div class="yiv1276106914MsoNormal">NABCEP<sup>TM </sup>Certified
installer Charter Member</div>
<div class="yiv1276106914MsoNormal">NYSERDA-eligible
Installer</div>
<div class="yiv1276106914MsoNormal">VT RE Incentive
Program Partner</div>
<div class="yiv1276106914MsoNormal"> </div>
</div>
</div>
</div>
<br>
_______________________________________________<br>
List sponsored by Home Power magazine<br>
<br>
List Address: <a moz-do-not-send="true" href="mailto:RE-wrenches@lists.re-wrenches.org" ymailto="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br>
<br>
Options & settings:<br>
<a moz-do-not-send="true" href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><br>
<br>
List-Archive: <a moz-do-not-send="true" href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br>
<br>
List rules & etiquette:<br>
<a class="moz-txt-link-abbreviated" href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</a><br>
<br>
Check out participant bios:<br>
<a class="moz-txt-link-abbreviated" href="http://www.members.re-wrenches.org/">www.members.re-wrenches.org</a><br>
<br>
<br>
<br>
</div>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
List sponsored by Home Power magazine
List Address: <a class="moz-txt-link-abbreviated" href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a>
Options & settings:
<a class="moz-txt-link-freetext" href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a>
List-Archive: <a class="moz-txt-link-freetext" href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a>
List rules & etiquette:
<a class="moz-txt-link-abbreviated" href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</a>
Check out participant bios:
<a class="moz-txt-link-abbreviated" href="http://www.members.re-wrenches.org/">www.members.re-wrenches.org</a>
</pre>
</blockquote>
</div>
_______________________________________________<br>List sponsored by Home Power magazine<br><br>List Address: <a href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br><br>Options & settings:<br><a href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><br><br>List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org<br><br>List rules & etiquette:<br>www.re-wrenches.org/etiquette.htm<br><br>Check out participant bios:<br>www.members.re-wrenches.org<br><br></div><br></div></body></html>