Insight on EKM vs utility discrepancy

Things we have found to be useful when installing or using our metering systems.
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Rownolds
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Joined: Mon Dec 24, 2018 7:51 am

Insight on EKM vs utility discrepancy

Post by Rownolds » Thu Dec 27, 2018 3:17 pm

TL;DR I have solar panels net metered. the EKM meter doesn't take into account the 1 kWh per day used by the solar inverters, but the utility meter does. So that's a source of 30 kWh per month discrepancy.

I have a 24 kW (2 inverters) grid tied solar installation I'm monitoring. I actually have 3 meters measuring electricity used/produced at several places - The utility meter measures and reports "to grid" and "from grid" directly (and thus net usage by subtraction), The (master) inverter measures from CT's placed between the utility and the main breaker (beyond which the solar panels feed in) and measures (internally) total solar panel production. Third one is the EKM which measures total consumption of the building (whether from the grid or from the panels. The EKM does not measure power fed to the grid at all). Total solar energy produced is typically 2000 - 4000 kWh depending on the season. Total usage is roughly 3000 to 4500 per month depending on season. There was a 50 kWh discrepancy between what the utility said the net energy was and what the EKM meter measured (corrected for net by calculating with total and 'to grid' figures from the inverter). The inverters each use 20 watts continuously, 24/7, so that's 30 kWh a month. This is 30 kWh of PV production that is measured by the inverters but the utility and EKM meters never see. So the discrepancy is really only 20 kWh, about 1%.
Just FYI

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