KWH meter discrepancy [RE-wrenches]

[John Berdner]

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.






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