I would always expect the power monitoring at the inverter ( or in this
case at the module) to be higher, because of wire losses. Matt's case
is about a 4 to 5% difference though, so that only explains maybe half
the loss at most. I wonder if the inverter monitoring measures incoming
watts, or before the transformers (if applicable), or other internal
equipment.....
Or is it more a case of "our monitoring is rated +,- 2% accuracy, so
we're going to calibrate it to +2%" to put a :) on all our readings?
Meanwhile I could see the utility metering being set on the low side of
their accuracy window to avoid fights with customers....
Have we never had an inverter Manu comment on this seemingly very
consistent discrepancy?
Has anyone with some seriously calibrated accurate metering determined
where all these differences are coming from? Cause I'm just guessing
it's a combination of the above factors, but I don't actually know.
I found a EPRI white paper saying 98% of utility meters are within 2%,
but I couldn't even find an accuracy spec for Enphase's monitoring.
Ray
/2011 9:11 AM, Bob-O Schultze wrote:
Good point, Matt. We've been required to install a revenue-grade solar
production KWH meter in Oregon with every system. In 6-7 years with different
inverter manus, I've never seen an inverter KWH display that wasn't, shall we
say... exuberant, in relation to the revenue grade meter.
Bob-O
On Jun 9, 2011, at 6:47 AM, m...@solar-energy-solutions.com wrote:
Mark,
We have nine Trina modules on our roof with Enphase. Enphase says the
array has produced 2,680 kWh to date. The revenue grade meter 12 feet
away says 2,564. Until I hear otherwise I am reluctant to consider
Enphase reporting precise. It seems to be similar to what I would expect
from a string inverter. I would be delighted to hear more anecdotes
confirming your observations.
Regards,
Matt
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