PV Series night time tare loss [RE-wrenches]

Wed Jan 28 07:01:34 PST 2004

Mark:

Thanks for paying for this report.  As you said, it's been a big
unanswered question by many folks for a long time, myself included.  I
have long supported and endorsed PV Series Inverters where appropriate.

Now, let's be honest about what it says in this report... I'm sure it
wasn't anybody's intent to mislead readers, after all.

First of all, it uses Inland Southern California as the basis for
production related calculations.  I'll buy that.  You gotta base it on
something.  Might as well use a best-case example.  Just be aware that
the loss percentages only go up from there, depending on where you are
located, and how your system is configured!

Secondly, the calculated "losses" are NIGHTTIME only.  This is only part
of the question that needs to be answered.  The tranny is hot 24/7 in
most applications and is a 24 hour a day parasitic loss.  Not, just at
night.  Just under double the 13 nighttime hours assumed in the report.
Using the example on Page 3 of the report, the nighttime tare losses are
2.85% of the production revenue.  Multiply that by 1.846  (13 / 24 =
1.846) to correct for 24 Hour Tare and you get 5.26% for that nice
optimized system in Sunny Inland SoCal.

The report also assumes a 23% System Capacity Factor.  I must assume
that is annual.  OK.  I'll buy that, but let's be "apples to apples" on
it.  This is a VERY optimistic factor for all but a tracking system (up
to 28 or 29%) or a "perfectly optimized" fixed installation in a very
sunny place!  Optimized includes: Due South, Lattitude minus 10 tilt,
Nil module mismatch, Nil wiring losses, and a fully loaded inverter.
Change any of these assumptions and the effective net % tare losses
grows rapidly.  For instance:  A large percentage of PV Series systems
are being installed on flat-roof applications, for instance... Right?
What kind of Annualized Capacity Factor do you think these systems are
averaging statewide in CA?  14%? 16%?  How about one in Livermore's
backyard... Santa Rita, for example.  

And if the inverter isn't fully loaded?  The tare loss is a constant
value.  Based upon the transformer, not on the actual PV KW driving the
inverter.  The constant 24/7 tare loss must be compared to the effective
AC KW baseline production estimates for each individual system and site.

Wouldn't a monthly / daily loss study / report be more useful for the
purposes of meeting the needs of those who actually have to design,
calculate production, sell, install, & warranty systems using your
equipment?  Remember, this is the very reason this question has been
asked for so long!  Who's left holding the bag when a system doesn't
meet production projections that were unwittingly based on "partial"
information?  (Gotta read between all the lines to get the whole story
from this report....)  What if that report didn't use up paper trying to
place $ value on the electricity (and "present worth factor????) and
just stuck to effective %'s?  What if that report included an easy to
read and interpret chart or table for various Capacity Factors or,
better yet, Slope / Orientation / Sun-Hours / KW DC combinations?  I
know that the report was generated to show "Xantrex's good side", so it
used best-case, but don't you have a lot of existing and burgeoning
markets elsewhere?

You stated that your competitors are exaggerating the issue.  Apples to
apples, all cards on the table, this report understates the issue. 

Just some food for thought... Lot's of Wrenches and others have been
thinking (wondering) and are hungry for reliable, useable info!

Pray for Sun & Higher Oil & Gas Prices!

Matt Lafferty
Universal Energies Institute
mlafferty at universalenergies.com
(916) 422-9772
(916) 628-7694 Cell
(916) 914-2247 Fax
www.universalenergies.com

-----Original Message-----
From: Mark Edmunds [mailto:mark.edmunds at xantrex.com] 
Sent: Tuesday, January 27, 2004 12:35 PM
To: RE-wrenches at topica.com
Subject: PV Series night time tare loss [RE-wrenches]

	Dear Wrenches, 

	There has been considerable discussion about the night time tare
losses incurred by the outboard isolation transformers used with our PV
Series of grid tie inverters.  In this forum, and in one of our
competitor's "white papers", we have seen some misleading numbers thrown
around.  We do acknowledge that the external isolation transformer does
lead to lost energy, however this loss has been exaggerated.  

	To assist installers in making informed product selections, we
hired a third party engineering consultant to study, measure, and report
on this issue.  We hope this helps you understand the issue, rather than
rely on unsubstantiated stories.  

	The report is called "Night time Tare Loss Cost Analysis".  You
can download the report here;
<http://www.xantrex.com/support/docserve.asp?id=823> .  

	Hope this helps clarify the issue.

	Thanks

	Mark Edmunds
	Director, Engineering
	Xantrex Technology Inc.

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