KWH meter discrepancy [RE-wrenches]
Matt Lafferty
mlafferty at universalenergies.com
Sat May 20 10:04:03 PDT 2006
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
-----Original Message-----
From: Kelly Keilwitz [mailto:kelly at whidbeysunwind.com]
Sent: Friday, May 19, 2006 2:27 PM
To: RE Wrenches listserve
Subject: Re: KWH meter discrepancy [RE-wrenches]
Hi Jeff & Bob-O,
Thanks for the responses.
All our grid-tie's require utility-grade meters for green-tag sales and,
now, for the WA State production incentive (modeled after Germany's Feed-in
tarif). After thinking about it several of our systems have inverter
metering as well as the utility meter. A GTFX 3048 system we recently
installed also showed more substantially more production on the mate than on
the (form 12S) utility meter. I chalked that up to issues with sorting out
the energy in, out and to the critical loads. But, maybe it's the same "con"
(conspiracy, confusion, confoundedness...?). I'm going to compare a Fronius
5100 output (nice display) with the utility-style production meter on a
recently installed system next week.
I find it difficult to believe that all of our utility- type meters are
inaccurate -- on the low side. Accuracy is what Austin & Hialeah are selling
with the reconditioned meters. What are inverter manu's selling?...
With your confirmation stories I'm satisfied that we didn't goof up somehow
with this meter.
Thanks,
-Kelly
On 5/18/06 5:48 PM, "Jeff Clearwater" <clrwater at earthlink.net> wrote:
>
> Hi Kelly & Bob-O,
>
> Yes, Since I install systems in MA sometimes and for all those systems
> we need to install "utility grade" KWH meters for the rebate, I too
> find the inverter's meter running 15-20% higher than the refurbished
> Austin Instruments meters like those bought through AEESolar or EO.
>
> And come to think of it the ones that were the most off were the 120
> VAC units (SB1800) though the 240 VAC units are off too though by
> varying percentages - usually around 15-20%.
>
> And that's a major drag as the MA rebate program depends on those KWH
> meters - not the inverters! So if they are low instead of the
> inverters being high - the folks are not getting their full
> performance portion of the rebate back.
>
> I recently installed two on some Windy Boys (120 VAC SB1800) and got
> 21% more on the inverters than the meters.
>
> For now I've stopped installing them - but need an alternative. They
> are nice in that they are cheap
>
> Anyone with a solution?
>
> Best,
>
> Jeff Clearwater
> Village Power Design
>
>
>
>> Hiya Kelly,
>> It may be the 120V thing on the 240 meter, but in Oregon we are
>> required to put a revenue grade meter in almost every installation.
>> Most are 240V, but some are wired 120V like you have depending on the
>> inverter. I've NEVER seen the meter from the inverter and the revenue
>> grade meter agree regardless of the inverter manu. And -here's the
>> salient part- the inverter meter is always the high one.
>> Uh huh.....
>> Bob-O
>> On May 18, 2006, at 10:02 AM, Kelly Keilwitz wrote:
>>
>>>
>>> Wrenches,
>>>
>>> We recently installed a small PV GT system with a SMA SB700, with
display.
>>> We also have a utility style "green tags" meter on the AC output.
>>> It's been running for a couple of months and the SB display is
>>> recording 18% higher energy production than the green tags meter.
>>> The green tag meter is a form 2S, 240-V meter, wired for the 120-V
output of the SB700.
>>>
>>> We've wired 240-V meters for 120V operation before but never had a
>>> (reliable) KWH meter to gauge against. Is the SB display meter
>>> reliable? Any problems with wiring 240V meters for 120V output?
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
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