Inverter meters; was Batteryless inverter recommendation [RE-wrenches]

Wed Feb 27 08:24:38 PST 2008

Thanks Jeff for bringing up those good points Matt came up with.

1. Check
2. Meter is on output of inverter only so, SB is about PF 1 right?
3. Check ditto above and only goes one way.
4. Check
5. One of them is still wrong.

Also there was talk a while back about some kWh meters not being very 
accurate below 200 watts, but I think that was mostly in mechanical 
meters being a little sticky, and this is a digital meter.

Brad
AEE Solar

Jeff Clearwater, Village Power Design wrote:
> 

> 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
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`~

Brad Bassett

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