Discrepancy between SolarEdge Production Meter and IoTaWatt

I’m seeing a consistent discrepancy between the SolarEdge revenue-grade production meter and the output I’m seeing from IoTaWatt of 3-4.5%. I have a SolarEdge HD WAVE SE11400H-US 11.4kW Inverter with 14.44kW DC of solar panels (38 x REC TwinPeak 72cell 380W panels). I’m using ModBus on the SolarEdge to get reading updates every ~5 seconds. The instantaneous readings are within 0.5% of the IoTaWatt (using a AcuCT-H063-100 CT) at low production levels and increase to around 1% when getting close to the 11.4kW peak output. However, the measured production in Wh is off by 3-4.5% (3% at low to moderate production and closer to 5% at high production). ModBus provides instantaneous output in watts as well as an aggregate of all power produced since installation, reported in Wh. When using Home Assistant to report output by hour and day using the Wh measurements from the SolarEdge inverter via Modbus, the Wh measurements exactly match the SolarEdge app/website, so there’s no issue there. What I’m seeing is that the hourly production appears to be higher than the peak instantaneous production at any point during that hour.

So, for example, when graphing the 5s instantaneous output info, if it shows a peak of 10.3kW, the production in Wh may show 10.6 kWh, which doesn’t make much sense. This only occurs with the SolarEdge. The IoTaWatt production numbers look accurate.

I have a smart meter from my utility that reports electricity imported from the Grid daily. The output, reported to the nearest kWh, has precisely matched import.wh from IoTaWatt (integrator grid = Main1 + Main2, output import = grid.pos). This seems to imply that my utility meter matches the IoTaWatt instead of the SolarEdge production as the solar should subtract from the electricity demanded from the Grid. For example, on 9/29, SolarEdge reported that I produced 69.9 kWh whereas IoTaWatt reported 66.8 kWh, a difference of around 4.5% and 3.1 kWh. IoTaWatt reported 20.4 kWh imported from the Grid and my meter reports 20 kWh, rounding down to the nearest kWh.

Each day since install has matched the utility meter:

9/29: IoTaWatt 20.4 kWh, Utility 20 kWh
9/30: IoTaWatt 25.2 kWh, Utility 25 kWh
10/1: IoTaWatt 33.6 kWh, Utility 34 kWh

So, I guess my question is why would the SolarEdge reading be so different from the CT in my electric box? It’s probably a 10’ run from the inverter to the CT, and from what I’ve read, electrical losses after the inverter should be on the order of 0.5-1%, not 3-5%. Documentation I had to submit to the county to get permit approval for the solar system shows they used 4 AWG wire to connect between the inverter and the AC disconnect (less than 10’) and from the AC disconnect to the electric box (3-4’ to line tap, 1-2’ to CT).

I’m also wondering if anyone else has seen something similar. I’ve been suspicious of the SolarEdge readings for a while, since they also seemed a bit too high given what I’m seeing from my utility. In addition, I did better than the HelioScope estimate, despite the fact that the estimate didn’t account for shading. However, before I had the IoTaWatt, I had no way to test these suspicions. I only have production monitoring on the inverter. The inverter is capable of production monitoring, but they never installed the CTs to enable that feature, and when I asked after the install was complete, it was going to cost around $400, more than I paid for the IoTaWatt, 2 200A CTs, 2 100A CTs, and 10 50A CTs.

It’s great that the IoTaWatt matches your meter, it typically does, but your question is about the solar production IoTaWatt vs. SolarEdge. You say that the IoTaWatt seems to match the Solar Edge instantaneous production readings, but is several % off on kWh.

IoTaWatt doesn’t record Watts. Rather, it maintains a running total of Wh and Time. Watts is computed as:
W = \Delta Wh / \Delta T.

So there should be no discrepancy between IoTaWatt’s Watts and Wh. This wouldn’t be the first time IoTaWatt has disagreed with other monitoring equipment, but it rarely disagrees with the utility meter.

Yeah, from what I can tell it’s the IoTaWatt that’s accurate here, given it matches the utility meter. I just find it odd that the SolarEdge meter, which is rated to be production-grade, is inaccurate. I can use the SolarEdge production figures to sell SRECs.

Somewhat unrelated, I’m currently using the AcuCT-H063-100 to measure the solar output. The contractors installed a 60A breaker so I wasn’t sure if a 50A CT was appropriate. In any case, should I expect measurements to be any different/more accurate with a AcuCT-H040-50? They’re both rated to 0.5% accuracy between 10-120% of the rated load but they appear to be accurate well below 10%.

There should be no significant difference switching to 50A.

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Is it possible that the inverter consumes some power itself, and is reporting on “generated” vs “sent along”?