Meters vs. Displays - Was: Batteryless inverter recommendation [RE-wrenches]

[Matt Lafferty]

Wrenches,

Great thread. There have been quite a few really good threads lately. Love
to see the participation and sharing of knowledge. I'll keep my post limited
to the topic of "inverter display" data vs. "generation meter" data.

The short answer to Jeff C's question about "any new info since May
'06?".... Yes. I've learned a &@!* load about different metering
methodologies and, perhaps more important than anything... Just how screwy
it is that, in this day and age, there are still huge gaping holes in the
standards for solid-state meters. Yes, this includes "Utility Revenue Grade"
models. I'll leave the muddy details out of this conversation. There is
ongoing work on that front by utility and related folks. (This affects
waaaaaay more than RE. In fact, RE is not even a flea on the back of this
issue...)  

There is also an in-progress development of testing requirements for
inverter integrated +/- 5% Class meters. For the foreseeable future, I
recommend installing at least a meter socket capable of accepting a
"standard" socket-type meter in the output circuit of all grid-connected PV
systems. Whether or not a generation meter is required by the utility or
incentive program. Whether or not you actually install a meter. At least put
the meter socket in there. It's cheap. I care less and less about whether or
not people take my advice with each passing day, so... Take that advice or
leave it. 

Another thing I have learned is how many brand new meters flunk utility
meter shop testing and are returned to the manufacturer for replacement
instead of being deployed. And those are self-certified and calibrated by
the mfrs. Some utilities physically test every single meter and some only
test sample quanitities. If the sample fails, then the batch fails. Along
the same line is something else I've learned, which explains a lot of
"unanswered questions"... The meters you get from Austin or Hialeah or other
such supplier, may or may not be fully tested and calibrated to the same
accuracy standard. In a nutshell, this means that apparently identical
meters installed in paralell on the same circuit, may or may not record the
same kWH readings over time. The differences might or might not be
significant, which in and of itself is subject to ones definition of
significant. The important thing to insist on is a certificate stating what
standard the meter you get is tested to. For "standard" socket type meters,
you're looking for ANSI C12.20 and an accuracy class. 

And, oh by the way, if you are installing one of the 3rd Party monitoring
systems that has an independent meter and they "calibrate" the meter data
feed to match the inverter display... Now you really don't know what the
truth is... But hey, this is solar after all, so maybe some don't care to
know the truth. Or maybe it doesn't matter in the big picture that I can't
see because, well, I care too much about the truth. Speaking of truth, I
don't think I've seen any module mfr reps on the list in like forever... But
back to metering. 

Joel D's SB2100 had approximately a 0.57% difference in kWH between the
inverter and the generation meter. (740.2 vs. 736 respectively.) This is
over a 5 month period. Assuming Joel's generation meter is fully certified
and calibrated, this is a reasonable discrepancy in my opinion.  Nightime
and standby losses being accounted for on the generation meter but not on
the inverter display could account for the entire difference. In a nutshell,
they may not actually be measuring exactly the same things.... Either way,
at least for that 5-month period, at Joel's place, the SB2100 kWH display
was nearly identical to his GE I70. (Joel, do you happen to have the
"cyclometer" style? According to the Maryland Public Service Commission, the
cyclometer style I70 is not recommended for some unstated reason... Maybe
they found out that is is calibrated to SMA SB2100 inverters!  LOL) Joel, I
hope you feel special. Nobody else's inverter displays match a socket-based
meter that closely over a 5-month period... I don't know which part of the
story I find more unbelievable... The part about the inverter display
matching the generation meter or the part about PV Watts being so close!
LOL  

Brad B & Ian W report "wildly" different and greater inverter display
readings than the utility generation meters.  Both of these inverters are
reported to be SMA SB700's. (120 VAC inverters with three output power level
ratings... 460, 600 & 700 Watts).  I suspect a couple of things here.

Brad & Ian: Are you comparing readings for the same time periods? You didn't
happen to turn the system on before the utility got there with a generation
meter, did you???  I know how we all hate waiting for them.... Brother Brad
did say it was "his system". This item was #1 on my original post from 2006
for a reason.  :-) 

Another issue is that a few kWH difference over a short period of time on a
relatively small system will equate to a large percentage. Comparing values
for identical periods over longer time will provide a more reliable basis
than just a few weeks. Also related to this is whether or not the generation
meters are "bi-directional". In winter months, it is common for inverters to
spend a greater part of their time in "standby", which can result in higher
percentage tare losses, thereby subtracting from the total generation.

Assuming the measured time periods were identical, the reported "wild"
difference smells like a software or configuration issue in the inverter
meter/display. In looking at the difference between the lowest & highest
output power ratings, I suspect the display is showing a value that is
calculated based on a configuration that is different from actual.  (For
instance: If the default setup is for a 700 Watt configuration, but you've
actually installed a 460 Watt, it could be possible that the value on the
display is calculated as if the system were larger than it is... Up to 52%
larger.) John B???

I won't mention names here, but there once was an inverter manufacturer who
shipped a few inverters with brains from other inverters... Which basically
ended up resulting in some wild differences between "expected" and
"measured" energy & power... Something with the magnitude of 75:125 or
100:225.... Don't remember the specifics.... Just the fact that the systems
actually ran, the installers didn't catch it... The 3rd party monitoring
company setup & calibration procedure resulted in data feeds that matched
the inverters' displays... It was a classic example of a mfr slip-up
followed by inadequate trained installers being directed to follow a flawed
3rd party procedure which caused a whole bunch of head-scratching, resorting
of the books, embarrassment all the way around, and wasted time by all. "So,
explain to me again why this 80 kW PV system is putting out 150 kW at 3 PM
in November..." Ooooopsie! The sick part is that certain folks thought it
just meant the systems were of "extra high quality". I'm sure we all
understand that these folks are the bosses... Advanced degree types... Not
Wrenches.... Like I said... I'm not gonna mention any names... 

Sunshine wishes to all....

>From the Hammock,
Matt Lafferty


-----Original Message-----
From: Jeff Clearwater, Village Power Design [mailto:jeffc at villagepower.com] 
Sent: Tuesday, February 26, 2008 5:31 PM
To: RE-wrenches at topica.com
Subject: RE: Batteryless inverter recommendation [RE-wrenches]


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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