Solar inverter self-consumption power is difficult to monitor and report.
Standard inverters are powered entirely by the solar generation, with the DC → AC being a one-way path. At night the inverter powers down. Hybrid inverters are responsible for the attached battery (usually lithium based) and stay on at night, so the power must come from somewhere else. My Solis inverter, I was told, always uses the power from the DC side as solar then battery, however testing shows that it still runs at night when the battery is disconnected, so clearly the power can come from the grid side.
Most of my system power readings are captured directly from the inverter. The standard power equation, for a solar and battery system, is
Solar + Import + Discharge = Load + Export + Charge
where the inverter reads the grid meter to provide grid Import and Export power, and the Charge/Discharge and Solar from its own internal monitoring or from the battery BMS. This just leaves ‘load’ as the unknown figure, and this is calculated by the inverter using the other figures as:
Load = Solar + Import + Discharge - Export - Charge
In a ‘steady-state’ situation, this equation must balance. Hence
S + I + D - L - E - C = 0
Like you, I find that my inverter is using about 30-40 Watts, perhaps up to 60 when the included UI screen is active. There are two questions
1 where is this power coming from (at any given point in time)
2 how it is being measured and reported (by the inverter)
When the sun is shining and solar PV is generated, the inverter is powered from the solar DC. This power use may or may not be included in the solar PV power reported by the inverter.
If the inverter power is taken from the solar pre-monitoring and reporting, then the power use is invisible and inconsequential.
If the inverter power is taken from the solar after monitoring, then the power use must show up elsewhere.
When the sun is not shining, and only for hybrid inverters, the inverter is powered either from the battery (as discharge) or from the grid. This will be dependent on how the inverter works or is setup.
The inverter power must then be either accounted for as being included in the load, or accounted for as being removed from the battery discharge, or added to the grid import, or not accounted for at all.
If the inverter power use is ‘p’
[S-p] + I + D - L - E - C =0
where true solar is S and reported solar is [S-p]
S + [I-p] + D - L - E - C =0
where the inverter power is added in with the reported grid import power
S + I + [D-p] - L - E - C =0
where the inverter power is added in with the reported battery discharge power
S + I + D - [L-p] - E - C =0
where the inverter power is added to the reported load power
In all the cases above the inverter power is compounded with one of the standard reporting figures for solar, battery, grid or load. However I suspect that the most common situation is that it is not reported at all, therefore (as all the above reduce to):
S + I + D - L - E - C = p
Hence the inverter power is the stable-state imbalance between all the inverter reported figures. This is exactly what I see, most of the time.
I have added a template sensor being the calculation, from the inverter-reported values
Delta (imbalance) = Solar + Import + Discharge - Export - Charge - Load
That is my ‘47 W’ shown bottom right.
In theory this should be 0. It is not, often due to timing differences between the reported figures, as not all power reading sensors update at exactly the same time. However when the load and solar generation are relatively stable, and not much else is happening, the difference given is the imbalance in the equation, which I assume to be the inverter power consumption. To be quite clear, this value (inverter self-consumption power) shows here simply because my inverter is not taking into account its own power use by subtracting it from any other figure that it reports.
This value could then be reported as consumption by a ‘device’ if you so wish (although I find it is far too unreliable and unstable as a figure for any practical use).
I would suggest that this is just not possible.
For standard (non hybrid) inverters, if most of the time the solar PV reported figure is the generated solar less the power used by the inverter, then the inverter power use is simply not available. If the solar reported is the true solar generated, then the inverter would have to report its own power use separately, otherwise this figure would only be apparent by separately and additionally measuring the actual energy passed out of the inverter. Most of us only record ‘solar’ as one figure, either reported by the inverter (true solar or solar after self consumption) or measured independently.
Where a hybrid inverter does continue to run at night, the power used may just still be ignored and not reported by the inverter. As you have already said, you have two systems one of which shows the power used, and for you the power can come either from the grid or from the battery, depending on settings. In the HA world, the ‘system’ is the ‘home’, and Solar Power and Grid are outside this - hence all solar power is inwards only. Batteries are also typically outside the ‘home’ and reported separately. This leads to the question - where is the Inverter as a device (and power-user)?
Inside the solar system - the power used is consumed before the solar generated is reported, and we have neither need nor ability to report it.
Inside the battery system - the inverter power can be attributed to battery discharge (except when the battery is not discharging) and probably then best subtracted from the true battery discharge.
Inside the home - the inverter power can be attributed as a separate device as part of or additional to the standard ‘house load’.
In my own case, I believe that my inverter shifts how it accounts for the internal power use during the day according to the situation, sometimes including this in the battery discharge figure, other times removing it from the solar generated figure, but most of the time just ignoring it. In practical terms I have no consistent way to measure the power consumption and report it, and I would suggest that ‘inverter power’ is simply best regarded as a loss of solar generation before reporting.
The same applies to Lithium batteries and the BMS. My battery stack burns around 15W running the BMS, and then there is also my Home Assistant etc… During December my solar system generates only around 35 kWh reported in total, but I believe it costs me 50 kWh to run everything. I should probably charge the batteries to 80% and turn the entire system off mid November for a couple of months.
