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difference between electronic meter and Sunny Boy Control values

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Posted by Melodie de l'Epine on August 25, 2004, 2:44 pm
We now have several examples on grid connected PV system of 1 to 3kWp
where there is a difference in the number of kWh measured by the
electronic meter installed by the utility, and the inverter.

for example:
electronic utility meter / SMA SWR700 inverter reporter by a Sunny Boy

june 03: 128 / 120
july 03: 135 / 120
september 03: 108 / 100
october 03: 57 / 50

the values are quite close, so it is hard to say which value is most
likely to be correct - and asking the utility to check their meter is
quite expensive, if the meter is ok.

And in another example, this time with a Gridfit 2200 (Total Energie)
inverter, the inverter reports nearly twice as much production as the
electronic utility meter (102kWh/fortnight as opposed to
66kWh/fortnight). In this case, the utility says that they have checked
the meter and that it is properly calibrated, BUT a quick simulation
shows that the production reported by the inverter is the most likely.

We also have seen several systems with both disk meters and electronic
meters (one private and one belonging to the utility) with, again, a
difference in favour of one or the other, generally a difference of 1 to
10 kWh/month

Until recently most systems here relied on disk meters for monitoring
production values, so we don't really have much experience with
electronic meters.

Could there be a problem with the inverters' built-in meters? Or are
electronic meters notoriously unreliable? Maybe disk meters have a
tendency to undervalue?

Any experience, insight, pointers etc would be helpful.


Posted by Solar Guppy on August 25, 2004, 3:14 pm
Hi Melodie,

Welcome to the word of Manufacture reported data ...

The SMA products report 2-3% high from my testing on over 5 different units
... I've directly spoken to them and they say the customers are "happy" with
how the units report data ... read the owner like seeing big numbers ..The
Xantrex Suntie is within 1% ...

Other manufactures an 2X worse in my testing , sorry , I can't release who
as I am under an NDA on the testing .. but I've done extensive testing on
Fronius , PVPowered , SMA and Xantrex  with lab grade power analyzers (.08%

Believe the Utility kWhr meter , it is the true measure of kWhr to the grid
... Most (not all) the manufactures report high 2-5% and don't care that
they do , it also helps support some of the units efficiency claims as well.
(hint: if it seems to good to be true , it is)


Posted by Melodie de l'Epine on August 25, 2004, 3:54 pm
 Solar Guppy a écrit :

OK, but.... the Suny Boy is telling us less than the utility meter! Are
they just being cautious? This seems to be the wrong way 'round!

Of course! but if that is the case, I'd like to know why the inverter in
the second recorded case is telling us such big numbers! Here we are
looking at a differnce of over 30%, compared to the inverter.

Maybe the inverters performance is suffering? The only thing I can think
of is that the inverter is reporting dc side kWh and functioning at a
very poor level of performance (around 70%), which could explain the
losses. The technical doc says over 90% efficiency at 20% of maximum power.

True, the GrdiFit inverter is a newcomer to the market, but still!!

Posted by John Beardmore on August 25, 2004, 10:54 pm
What exactly is being measured ?  Watts ?  VA ?

Could power factor be an issue ?

Cheers, J/.
John Beardmore

Posted by Melodie de l'Epine on August 26, 2004, 8:04 am
 John Beardmore a écrit :

The Gridfit manual says it displays kWh - we are measuring & comparing
production, and not power.


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