KWH meter discrepancy [RE-wrenches]

[Jeff Clearwater]

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Hi Bill and all,

Thanks so for this!   If I understand Jeff N. correctly the utility 
meters are much more to be trusted than the invertter readouts.  Am I 
understanding this correctly?

As I said in the last post and as Jeff N. correctly predicts below - 
the reverse-forward calibration is not it.  All of the systems are 
simply wired in series with the output of the grid-tied inverter - 
they only go forward - nothing to do with house loads or reversing 
here.

So John and other inverter manufacturers.  How much of the production 
of the inverters are harmonics?  Is this included in how the meters 
read?

Makes for a hard situation for us installers that interface with the 
customers and the performance based incentive programs - How do we 
tell them to ignore the meter on a $2000 device cause it's 15% off? 
Seems like real power back to the grid is what we are looking for 
here.  Why measure or report anything else?  With our industry so 
dependent on predicting accurate payback and economics - seems to me 
we can't afford to have the inverter's internal metering (and thus 
what is reported through ethernet and modems to web sites etc.) be 
off 15%!

Thanks again to Bill (and pass on our thanks to Jeff N.) for getting 
closer to the bottom of this!

Need to hear from the inverter manufacturer's next on this I think.

Best,

Jeff C.





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>Jeff and all,
>
>My esteemed colleague, Jeff Newmiller, who is not a member of this group,
>put together a very nice reply, and I think his explanation is relevant and
>the most plausible at this point. His message follows.
>
>****Jeff Newmiller's message:
>I think Matt Lafferty is on the right track in this discussion. I think the
>reverse-power-flow-calibration can be an issue, but it is difficult to
>compare net-metered readings with inverter output readings because of house
>loads so I tend to think this not where the discrepancies are showing up.
>Rather, it is where there are redundant energy production meters which are
>wired for forward power flow that the discrepancies will be most easily
>noticed.
>
>1547 goes to noticeable pains to describe referencing the harmonic current
>content of an interconnecting device to its rating, instead of against the
>measured fundamental current.  This is because they recognized that the
>absolute amplitude of current harmonics is the most important information in
>determining the effect on grid power quality, not the relative fraction.
>
>This means that compliant interconnecting power systems are allowed to
>produce just as much power in harmonics at low power as they would at high
>power.  Measurement of energy based on
>fundamental+harmonics will therefore be larger than energy measured
>fundamental+based
>on I*V*PF or by meters that internally suppress harmonics (intentionally or
>not). For example, a 1kW inverter reporting 100W output may be producing 60W
>of 60Hz power and 40W of harmonic power at the third harmonic yet still meet
>IEEE1547 (4% maximum single harmonic referenced to current level at 1kW).
>While this scenario is unlikely, it shows that being off by 66% is allowable
>so a 15% difference should be quite possible based only on a "include
>harmonics" versus "fundamental only" difference in type of measuring device.
>
>Since the power contained in harmonics is far more likely to be dissipated
>uselessly than the power produced at 60Hz, I don't think it is fair to
>include "all harmonics" power when selling energy into the utility grid.
>However, depending on how an inverter manufacturer implements their energy
>accumulation, they may end up doing just that.
>
>----------------------------------------------------------------------
>Jeff Newmiller, Project Engineer                       BEW Engineering
>mailto:jeff.newmiller at bewengineering.com http://www.bewengineering.com
>----------------------------------------------------------------------
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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

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425 Nimrod St.
Nevada City, CA 95959
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