KWH meter discrepancy [RE-wrenches]

Tue May 23 09:32:06 PDT 2006

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Hi Matt and All,

On 5/20/06 12:04 PM, "Matt Lafferty" <mlafferty at universalenergies.com>
wrote:

> 
> 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.

Our customer-owned utility-grade PV production KWH meters go into service
when the inverters do. The (separate) Utility-owned net meters usually take
awhile to get installed by the meter tech, though.

> 
> 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...

Thr KWH meters are on the inverter output, backfeeding a breaker in the
house system. Why should the meter be affected by the power factor of the
house loads or site (except, perhaps, for battery backup systems)?
{This is probably where I should simply say "Awww shucks" and "Golly gee"
and "I didn't know that... Wow!"} :)

> 
> 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.

If inverter energy consumption at night is significant it seems to me that
the (inverter) meter should account for it. Or, at least have a "net total
production" reading.
> 
> 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.

The meters we use are "120/240-V" For 120-V operation the hot leg is run in
series through all 4 jaws, the small latch on the back of the meter is
opened and connected to neutral. However, I don't know if the accuracy of
the meter is guaranteed in the 120V configuration. I'll check on this....
> 
> 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.

This makes sense, but shouldn't account for 15 to 20% differences, should
it?

> Here are the high-points of my testing and research over the years:

Thanks for the input, Matt. I can see that, like many things that we do, the
answer isn't as simple as we would like it to be. I'll compare outputs
between the meters on a few grid-tied systems that have been running for at
least a year and post the results to the List.

-Kelly
Kelly Keilwitz, P.E.
Whidbey Sun & Wind, LLC
Renewable Energy Systems
NABCEP Certified PV Installer
987 Wanamaker Rd, 
Coupeville, WA 98239
PH & FAX 360-678-7131
sunwind at whidbeysunwind.com

The rest of what Matt said:

> 
> 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.

> 
> Matt Lafferty 
> Universal Energies Institute
> mlafferty at universalenergies.com
> (415) 252-0343 Phone
> (916) 914-2247 Fax
>  

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