KWH meter discrepancy [RE-wrenches]

[Jeff Clearwater]

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Hi John, David, Kirk, Mark, Bob,  Bill  and all,

The meters I am reporting are all wired so that they record only the 
forward output from the inverter.  So the backwards explanation is 
not it - at least not for these cases.

David and Bob - you've certainly bought enough meters from Austin to 
warrant some testing on their part perhaps?  Or perhaps an accuracy 
curve at the various wattage levels?  Present the problem to them and 
see what they say?

And John you say at low power it's difficult to measure accurately - 
so is your +/- 4% - is that at full power?  Is there any sense of 
accuracy for a typical PV cycle?

Thanks again!

Jeff C.

>Wrenches:
>
>Regarding meter accuracy in both directions....
>I spoke to a guy in PG&E's meter reading department some time ago.
>He said he had to check about 30 meters (the fancy electronic ones) to
>find one that met accuracy spec's in both directions.
>All met spec in the "forward" direction but only a few met it in both
>directions.
>This is not altogether surprising to me since bi-directional metering
>is relatively new and accounts for double O squat of the totl number of
>units sold.
>
>For Sunny Boys our accuracy is +/- 4% and we usually try to calibrate
>the inverter for 0 to +2%.
>At low power it becomes difficult to measure accuractly.
>This is true for all measurements not just inverters.
>You normally specify accuracy at full scale and so at low power a small
>full scale error becomes much bigger.
>This can lead to higher error if the system frequently operates at low
>power.
>
>I still don't think these two items explain the circa 15% errors some
>are reporting but hopefully it helps shead a little light.
>
>Best Regards,
>
>John Berdner
>
>
>
>>>>  bill at brooksolar.com 5/21/2006 07:03:58 PM >>>
>
>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.
>
>
>
>
>
>
>--
>[Non-text portions of this message have been removed]
>
>
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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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