Batteryless inverter recommendation [RE-wrenches]

Jeff Clearwater, Village Power Design jeffc at villagepower.com
Tue Feb 26 17:30:35 PST 2008


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Hi Brad,

At 1:20 AM +0000 2/27/08, Brad Bassett wrote:
>I now have a few weeks of time on my SB700 system that also has a
>utility owned digital revenue meter on it. So far the SB meter reads
>145% of the revenue meter reading.


I've brought this to the attention of the List in the past (See 5/18/ 
- 5/20/06 - KWH meter discrepancy).  Matt replied with a very 
complete list of possible causes (below).

We do really need to get to the bottom of it.  Any more thoughts or 
updates since May of '06 Matt?  Others?

Best,

Jeff Clearwater
Village Power Design

At 10:04 AM -0700 5/20/06, Matt Lafferty wrote:
>Wrenches,
>
>I see a couple of things "missing" from the descriptions in this thread for
>a conclusive determination of the true cause for the discrepancies...
>
>1) Hours of operation.  In many cases, even though it is generally
>"prohibited", the PV systems will have a number of hours or days
>"head-start" on the PV Generation Meters.  I've seen numerous cases where
>this accounts for over a month of production on the inverters by the time
>the PV Meter is installed.  If your comparisons have been made on relatively
>new systems, there is a good chance some of your discrepancy is related to
>this.  It is important to take a reading on both meters at the same time,
>then take another reading some time later for proper comparisons.  The
>longer the time interval between readings, the more accurate your comparison
>results will be.  (I'm not saying they will be identical by any means.)
>
>2) Most revenue grade kWH meters account for power factor losses to some
>degree.  What this means is that a site with a low power factor actually
>"uses" more energy to do the same "work" as far as the meter is concerned.
>This is a double-bite for our purposes... The Customer is paying for kWH
>that they wouldn't need if the PF was 1.00 and their PV Meter is recording
>the opposite effect. This occurs regardless of whether the low power factor
>is "incoming" or "site-caused. I can't speak for the Inverter Manus, but my
>observations, testing, and comparisons have led me to the conclusion that
>their metering does not account for this as accurately as revenue grade kWH
>meters.  (Note: It is common for utilities to deny this and/or claim that
>the low PF is caused by the Customer.  I've seen them claim that they are
>"losing revenue" because of this issue.)
>
>3) Most revenue grade kWH meters are very accurate in the "forward"
>direction (+/- .5%), but wildly vary in their accuracy when running
>"backward". This aspect is huge on a number of fronts and there isn't room
>to describe the plethera of scenarios here. You may be wondering "why would
>my PV meter run backward?"... Simple.  At nite, the inverter uses some AC
>power... A very small amount based on the Manus, but some nonetheless.  It
>is impossible to tell here what that effect has on the "dial readings".  The
>Inverter displays do not count this.  For starters, you will have to know
>whether or not the PV meter is detented or non-detented.  After that you
>will have to find out whether or not it is calibrated to meter
>"bi-directional" or not and the accuracy tolerances for both.  Good luck
>getting that info!
>
>4) If you have a 120V Inverter, you need a 120V meter to properly meter it.
>Running the noodle through one leg of a 240V meter will not provide accurate
>results.  Period.
>
>5) Electro-mechanical vs. "calculated".  This is another spot where the
>differences get wild. Your PV Meter is most likely EM.  Inverter
>displays/meters are calculated.  The circuitry & programming in the
>Inverters uses "constants" in order to "do the math".  Contact your Inverter
>Manu for specifics related to their methodology.  Be sure to let us all know
>what you find out!  (Hint: This is very closely related to #2 above.)
>
>Here are the high-points of my testing and research over the years:
>
>After eliminating #1 & #4 above, the largest discrepancy factor is a
>combination of #2 & #5. Once you've isolated and quantified them, multiply
>the Inverter meter reading by the average Power Factor.  You will be amazed
>how closely the result comes to matching the PV Meter reading.
>
>How to go about doing it:
>
>In order to track this, you'll have to get your hands on a recording
>analyzer that does power factor measurements.  (I.E. Fluke 43 or similar...
>~$2K) If you don't have one, and most of us don't, it is possible to request
>a "power quality" or "meter accuracy" test from your utility company. 
>
>Generally they all do this at no charge.  You will have to tell them
>specifically what aspects you want results for and the interval that
>measurements are taken... Try to talk directly to the Power Quality Tech and
>let him/her know what you are trying to do.  They are generally pretty laid
>back and quite helpful if you aren't pushy or "blaming the big bad utility".
>They will let you know what they can measure/record at what intervals and
>for how long.  Generally speaking, the more channels you want measured
>reduces the length of time and increases the interval period for your test.
>After you get a decent relationship going, be sure to try to get the "data"
>as well as the standard "report".  Ask for it in a CSV or spreadsheet
>format.  This may or may not be possible due to "policy".  Generally
>speaking, you will get a lot more bees with honey, so be sure to have jelly
>donuts on hand.  Your attitude will play a large part in determining how all
>this goes. If you know you can't be civil with them, send somebody who can!
>(Hint: "Awww shucks" and "Golly gee" and "I didn't know that... Wow!
>Thanks!" will get you a long ways toward your goal... These guys are
>"tech-heads" by nature and they can talk way over your head in most cases...
>Way over mine, anyway. The "Awww shucks" thing plays nicely in getting the
>conversation onto a "plain English" level.)
>
>Some utilities have power quality recorders available to Electrical
>Contractors on a loaner basis for use in their service territories.  Check
>into this option.
>
>OK. Off to spend the rest of the weekend campin at a Hot Springs.  Have a
>good one, ya'll!
>
>Pray for Sun!  (I'd prefer not to have my sleeping bag soaked)
>
>Matt Lafferty
>Universal Energies Institute
>mlafferty at universalenergies.com
>(415) 252-0343 Phone
>(916) 914-2247 Fax
>




>I now have a few weeks of time on my SB700 system that also has a
>utility owned digital revenue meter on it. So far the SB meter reads
>145% of the revenue meter reading.
>
>Brad
>AEE Solar
>
>
>Joel Davidson wrote:
>
>>
>>  I have 20 Siemens SP75 modules feeding into an SB2100U inverter and a GE
>>  I70
>>  kWh meter. From September 20, 2007 to February 22, 2008 the SB2100U
>>  measured
>>  740.2 kWh and the GE I70 measured 736 kWh. (PVWATTS estimates 748 kWh at
>>  81%
>>  DC-to-AC derate.) Looks good to me.
>>
>
>
>
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-- 
~~~~~~~~~~~~~
Jeff Clearwater
Village Power Design
Sustainable Energy & Water Solutions for Home & Village
http://www.villagepower.com
gosolar at villagepower.com
NABCEP (tm) Certified Solar PV Installer

Voice: 413-259-3776
Fax: 413-825-0703
65 Schoolhouse Rd
Amherst, MA 01002
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