[RE-wrenches] Warranty Claim Based On Enphase Monitoring Data

[Ray Walters]

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, matt at 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
>