question

@denniz What grid meter are you using? IF a ET112, they have high latency therefore when the calculation is carried out between the CT sensor on the AC in of the Multiplus, and the grid meter, the calculation can be out. The CT sensors have low latency but are not the most accurate during low loads, whereas the ET112 has high latency but is very accurate. This difference in performance between the 2 measuring devices can create difference in readings. Other things to consider are the efficiency of the charger (rule of thumb 95% efficient) and the inverter (rule of thumb 10% efficient). If you are charging the batteries, then running the inverter to power a resistive load, you could end up with 15% round trip losses.

Again, assuming you have a ET112, I generally ignore the data from that because the power (not energy) reported in VRM for example is out on my ET112 compared to my actual grid meter all the time.

Other factors to concider, what is the charge efficiency of your batteries. My first ESS/ UPS system was with lead acid batteries. I had 40% losses on those because of the crap charge efficiency of the batteries, the crap battery charger I was using and then the inverter was just rubbish with a 15% loss on DC to AC.

Hi Jason,

Thanks for the info to consider. The next step might be to acquire something like the ET112. I am actually just comparing the Multiplus CT sensor to my "utility company" meter. Actually the meter built into the dock pedestal at my marina. Good to know about the high losses on the lead acid batteries. I'll certainly be switching to lithium once they are EOL.

Right now, however, I am not using the batteries for much of anything. They run a handful of LED light fixtures for a few hours each evening and my sink water pump occasionally. I will try to take some measurements with the charger disabled.

The problem that I am seeing is very high usage measured on my utility company energy meter that does not reflect what my anticipated usage would be. In my simple overnight test outlined above, there is a large discrepancy between the 25kWh measured by my energy supplier, the 9.16kWh measured by the Multiplus and the expected maximum usage of around 15kWh.

There should not have been any inverter usage at this time. Multiplus is configured to charger only. I do not think that it could have put 10kWh into the batteries during that time simply because they were already topped up and are rarely used. I admit I do not have a way to verify that beyond what is being reported by the Multiplus. Next time I will try with the charger disabled.

So this leaves at least 1kW of power unaccounted for 9.5 hours, even if we assume the Multiplus energy readings are inaccurate and just assume worst case scenario for the single 1.5kW resistive load over 9.5 hours. Could a fault somewhere in the Multiplus be consuming 1kW? The transformer? Incorrect wiring?

The only other symptom I have is that the unit hums rather loudly when connected to grid power regardless of any AC output load. High load, low, load, no load, it will still hum and the intensity varies over time even if the charger and output are both disabled. So that leads me to believe that something about the input power is causing the change in intensity, or something internal to the Multiplus itself. Could those same factors be causing the increased power consumption? Perhaps leakage current somewhere? I want to be sure nothing is wrong with the unit that could be using excessive energy before trying to request a new grid meter.