Large Discrepancy beetween Self-Consumption and Self-Sufficiency

[conanthebarber]

Hello alll!

Maybe somebody can explain what is going on here. In the readings of my mobile app I have a large difference beetween Self-Consumption and Self-Sufficiency. And that is constant, not only on one day.

My Setup:
Inverter: H3-12.0-E
Storage: 3x ECS 4100

My self sufficiency is only about 60 % of my self consumption. Some Examples
Septemver 2024 (Month) Sell-Consumption 280 kWh, Self Sufficiency 143 kWh, Grid Purchase 100 kWh
October 2024 (Month) Sell-Consumption 306 kWh, Self Sufficiency 192 kWh, Grid Purchase 73kWh
Januar 2025 (Month) Sell-Consumption 224 kWh, Self Sufficiency 122 kWh, Grid Purchase 127 kWh

Does this mean that the efficiency is only about 60 %?
And if yes: How is that possible? The claimed efficiency of inverter (97 %) and storage (> 95%) do not justify these values.

[Dave Foster]

There isn't enough information there to calculate -

If you use the V2 app and look at October 2024 can you post your 'Production', 'Export', 'Grid Consumption' and 'Grid Purchase' values please (see image)

Can you also confirm whether you have any other sources of solar generation, additional inverters etc...

[conanthebarber]

I can also post the other values

September 2024 (Month) Production 970 kWh, Export 689 kWh, Sell-Consumption 280 kWh, Self Sufficiency 143 kWh, Grid Purchase 100 kWh, Consumption 244 kWh
October 2024 (Month) Production 448 kWh, Export 141 kWh, Sell-Consumption 306 kWh, Self Sufficiency 192 kWh, Grid Purchase 73kWh, Consumption 265 kWh
Januar 2025 (Month) Production 235 kWh, Export 11 kWh, Sell-Consumption 224 kWh, Self Sufficiency 122 kWh, Grid Purchase 127 kWh, Consumption 250 kWh

The ratios look about the same in the other months.
There are no additional inverters or solar panels or other power sources.

brgds
Sven Weiberg

[Dave Foster]

ok, thanks - i've got a spreadsheet that I drop those numbers into and it produces this which hopefully explains the calculations

September 2024
Your solar Production was 970.0 kWh (A)
You Exported 689.0 kWh (B)
So your Self-Consumption is (A-B) = 281.0 kWh (C)
Your Self-Consumption ration is (C / A) = 29%

Your consumption was 244.0 kWh (D)
You Purchased 100.0 kWh from grid (E)
Your Self-sufficiency is (D-E)=144.0 kWh (F)
Your % self-sufficiency is the ration between (F / D) =59%

Your system efficiency (Input/Output) is:
Input = Production (A) + Purchase (E) i.e. 970.0 + 100.0 = 1,070.0kWh
Output = Export (B) + Consumption (D) i.e. 689.0 + 244.0 = 933.0kWh

Your system losses is the difference between Input and Output (137.0kWh ) which is 12.8% of the Input ~ 87.2% efficient.

October 2024
Your solar Production was 448.0 kWh (A)
You Exported 141.0 kWh (B)
So your Self-Consumption is (A-B) = 307.0 kWh (C)
Your Self-Consumption ration is (C / A) = 69%

Your consumption was 265.0 kWh (D)
You Purchased 73.0 kWh from grid (E)
Your Self-sufficiency is (D-E)=192.0 kWh (F)
Your % self-sufficiency is the ration between (F / D) =72%

Your system efficiency (Input/Output) is:
Input = Production (A) + Purchase (E) i.e. 448.0 + 73.0 = 521.0kWh
Output = Export (B) + Consumption (D) i.e. 141.0 + 265.0 = 406.0kWh

Your system losses is the difference between Input and Output (115.0kWh ) which is 22.1% of the Input ~ 77.9% efficient.

January 2025
Your solar Production was 235.0 kWh (A)
You Exported 11.0 kWh (B)
So your Self-Consumption is (A-B) = 224.0 kWh (C)
Your Self-Consumption ration is (C / A) = 95%

Your consumption was 250.0 kWh (D)
You Purchased 127.0 kWh from grid (E)
Your Self-sufficiency is (D-E)=123.0 kWh (F)
Your % self-sufficiency is the ration between (F / D) =49%

Your system efficiency (Input/Output) is:
Input = Production (A) + Purchase (E) i.e. 235.0 + 127.0 = 362.0kWh
Output = Export (B) + Consumption (D) i.e. 11.0 + 250.0 = 261.0kWh

Your system losses is the difference between Input and Output (101.0kWh ) which is 27.9% of the Input ~ 72.1% efficient.

On the efficiency Fox quote a number of efficiency numbers, but the act of converting power from the grid into dc to charge the batteries, and then storing it (which is 97%) then reversing the conversion process has more like a 15% loss - you have MPPT losses and various conversion points plus the power needed to run the inverter, bms and batteries.

All of that said your September was 87% efficient which isn't bad for a 3 phase, my single phase inverter did just over 90% in the same month.

[conanthebarber]

Sorry. That doesn't add up for me.
I think power that I consume directly from the grid cannot be part of the efficiency calculation for the inverter and storage. The power that I consume directly from grid should not be reduced by the converter. So imho it cannot be part of the efficiency calculations for inverter and/or storage.
The explanation in the app is relatively clear: "Self-consumption shows how much PV yield is used for home load and battery charging" and "Self-(Sufficiency) shows how much load consumption is taken from PV and battery discharge"
In the end the numbers mean that from every 1 wH pv yield that is not exported to grid only about 0.6 wH are consumed in my home. The other 0,4 hW are lost. (Or am I wrong here?) And that seems to less to me. Either the efficiency of the inverter and storage is worse than specified in the data sheet (97% inverter, > 95% battery) or the power needed to run the inverter, bms and batteries is about 4 kWh each day. Both would be bad.

But thanks for the answer!

brgds
Sven

[Dave Foster]

Sorry. That doesn't add up for me.
I think power that I consume directly from the grid cannot be part of the efficiency calculation for the inverter and storage. The power that I consume directly from grid should not be reduced by the converter. So imho it cannot be part of the efficiency calculations for inverter and/or storage.

That would only be true if your grid consumption is never used to charge your batteries, however if you wish to assume grid purchase is 100% transparent you cannot remove grid purchase from the efficiency calculation without removing it from consumption as well.

if you re-run the stats for September and assume your grid purchase of 100kw is 0, then you must remove 100kw from consumption (244-100=144kw) which then gives:

September
Your solar Production was 970.0 kWh (A)
You Exported 689.0 kWh (B)
So your Self-Consumption is (A-B) = 281.0 kWh (C)
Your Self-Consumption ration is (C / A) = 29%

Your consumption was 144.0 kWh (D)
You Purchased .0 kWh from grid (E)
Your Self-sufficiency is (D-E)=144.0 kWh (F)
Your % self-sufficiency is the ration between (F / D) =100%

Your system efficiency (Input/Output) is:
Input = Production (A) + Purchase (E) i.e. 970.0 + .0 = 970.0kWh
Output = Export (B) + Consumption (D) i.e. 689.0 + 144.0 = 833.0kWh

Your system losses is the difference between Input and Output (137.0kWh ) which is 14.1% of the Input ~ 85.9% efficient.

[conanthebarber]

"That would only be true if your grid consumption is never used to charge your batteries"
Of course grid consumption should not be used to charge my batteries! That's the idea of the "self-use" mode I selected. Only solar power not exported to grid should be used to charge the batteries. That's how the manual describes this mode:

"Self-use (with PV Power)
Priority: load>battery>grid
The energy produced by the PV system is used to optimize self-consumption. The
excess energy is used to charge the batteries, then exported to gird.
Self-use (without PV Power)
When no PV is supplied, the battery will discharge for local loads first. Battery will
charge when excess generation from other generation sources is detected."

So I think it is correct to only regard Self-Consumption when calculating the efficiency.

[Dave Foster]

You can read the manual or understand the facts, it’s up to you.

Also irrespective of the workmode you are in the batteries, the BMS, inverter and batteries consume standby power (from the batteries), and once the battery reaches minsoc it will float charge from grid to maintain its minimum soc.

[conanthebarber]

I just stumbled over a study from a german university (htw berlin) that tested the foxess h3 10 with 6x ECS 2900 and found a hint there.
Allthough foxess got a good general result there it looks like the efficiency with a consumption load < 500 W is really bad (40 % to 70 %)
That would explain my results. The load is very often below 500 w, especially in the night.
So the inverter should better not be used in usual home environments!
This also looks like a problem with some other inverters, so it is not only foxess having that problem.

For those able to read german, here is the link:
https://solar.htw-berlin.de/studien/str ... tion-2025/