I managed to fully charged the battery yesterday so decided to do a capacity test by fully discharging.
It was actually at full charge a few times so I used some loads to both test the inverter with 2800W+ for about an hour or two and use the available solar to heat the house.
Then I stopped the loads a bit to late to get again to full 100% SOC but 97% was close enough to start the test and the 3% delta will be considered at the end.
You can see the energy counters just before resetting and the fact that I got just over 14kWh from the 3.1kW PV array
as seen from solar graph there where a few clouds but not bad and peak was around 2900W
The start point at 97% with a load around 900 to 1000W (a resistive heater).
I took an one hour pause from discharging here so that discharge will end at a more reasonable hour in the morning.
The inverter idle is quite bad at 55 to 60W but other than that is a great inverter and very capable.
Test was restarted here.
So test ended probably at around 4.9% or so with the declared capacity as 540Ah witch I was already considered as conservative.
The energy counter was exactly at 500Ah about 13kWh but we need to add the 3% deficit at the start so 515Ah or 13.4kWh usable capacity in the 3.55V to 2.8V range.
There may be a max 1 or 2% due to small imbalance as pack did not had time to properly top balance as it was just the first full charge since installed.
And there may be 1 or 2% if discharged to spec 2.5V and charged to 3.65V but is a bit of a stretch to get to 280Ah.
In any case I'm more than happy with the results as the capacity is in range and less relevant than degradation over time so real test will only be available in 1 to 3 years when I can observe the amount of cycling and calendar degradation of this cells.
Now the attempt to charge the pack as the sun just rised.
Discharge was enable somewhere between 7 and 11% when all cells exceeded 3.2V but I manually turned off the inverter to give the best chance of having the battery charged before the end of the day.
Unfortunately just about 10kWh where available today due to clouds so no full charge.
Will try to do a more controlled test in January or February when day is longer and I can do also a full charge.