KWH meter discrepancy [RE-wrenches]

Sun May 21 19:01:14 PDT 2006

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Hi Bill,

Fact is we've seen this on numerous occasions - all consistently 15% 
lower than inverter readings.  No improper programming or damage in 
shipment involved.  I've seen it perhaps 15 times now.

When we are talking low power - we mean the amount of time spent at 
perhaps 10-100 watts.  The effect has been most pronounced on a 
Whisper Wind link where the machine spends alot of time on coastal 
daily wind patterns where it's just above start up speed and 
producing 10-50 watts.  Wind the storms come in it puts out 1000 
watts but that's rare compared to the amount of time it spends at low 
wattages.  This meter reads 21% under the inverter readings.

PV systems also spend some considerable time putting out wattages of 
this level - during morning and evening conditions and cloudy periods.

So if you are so sure it's not the meters then we really need to hear 
from the inverter manufacturers here!  Or utility meters are not that 
accurate - one or the other or a combo of the two has to be true.  To 
say that it just can't be true is to ignore the data.

Let's get to the bottom of this!

Best,

Jeff C.

>Jeff, Bob, and All,
>
>Bob and I have briefly discussed this in the past. They have seen
>discrepencies, and I believe they were due to improper programming or damage
>in shipment. The idea that a utility meter is inaccurate at low power just
>can't be true. A residence lives at 200-800 watts for over 12 hours a day
>considering night and midday. They are tested at several points including
>low power. All utilities retest their meters and occasionally they reject a
>meter for being out of calibration. Net meter meters require testing several
>meters to find one that is in calibration in both directions. However, the
>specs are so tight (I believe they have to be within 1.5% in both
>directions), that it is tough for a meter that has been calibrated only in
>the forward direction.
>
>Utility meters calibrated by Austin Intl. should be extremely accurate
>unless it was damaged in shipping. The specs are more stringent than
>inverter manufacturers. However, I have seen several inverter meters in very
>good agreement with well-calibrated utility meters, so not all manufacturers
>push the truth.
>
>Bill.
>
>
>-----Original Message-----
>From: Jeff Clearwater [mailto:clrwater at earthlink.net]
>Sent: Friday, May 19, 2006 8:05 PM
>To: RE-wrenches at topica.com
>Subject: Re: KWH meter discrepancy [RE-wrenches]
>
>
>
>Hi Bob and all,
>
>Yes that has been my theory as well - mostly because I trust digital
>internal inverter metering - and because the effect does seem to be
>pronounced on systems that spend alot of time in the lower wattage
>realm (like a Whisper wind Link that's always just over startup
>speeds).
>
>But I must say my trust of the inverter companies internal metering
>here is in question - does seem funny that utility grade meters would
>be that far off.
>
>Either way we really need to solve it as more and more state rebate
>programs (MA, WA) now require a "utility grade" meter and are based
>on performance - and the customers love EZ read meters over something
>complicated or digital or menu driven.
>
>How about a call to Austin and research it with them - maybe they
>have an idea or a higher grade meter or ???
>
>Any other simple approved meters out there?
>
>Thanks!
>
>Jeff
>
>
>
>
>
>>Jeff and all,
>>
>>We've seen exactly the same results with most meters we've tested. 
>>It's just a thought in the back of my mind and have not proven this,
>>but I suspect residential utility grade meters in general may have a
>>"sweet spot" in accuracy based on the average residential loads. 
>>Perhaps they just aren't accurate at the low power levels we see from
>>pv early morning, late afternoon, and cloudy weather?
>>
>>Best Regards,
>>Bob Maynard
>>Energy Outfitters
>>
>>
>
>
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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

530-470-9166
877-SOLARVillage
877-765-2784
72 Baker Rd.
Shutesbury, MA 01072
425 Nimrod St.
Nevada City, CA 95959
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