Youd want to get in an independent expert or firm in to provide a report.
If you can find anyone willing to do that and it backs up what you are saying that'll be all you would need.
Yeah I remember ronski linked a company on here once that might fit the bill.
Next time I am on site I am going to discuss with my works electrical manager.
I believe the way you would normally test battery capacity is to set a fixed known draw. Say 100w and leave it to draw that constantly and then time x current will give you kwh.
I dont think however thats easy to do with these since you dont want to fully drain them, so you need to attach the BMS, and its difficult to control the BMS. (for mine BMS is a standalone unit)
I can show plenty of screenshots of their own portal showing :
1) repeatedly charging at 5.5kw the batteries charge for around 1hr 40 mins before dramatically cutting the power and stopping by around 1hr 50 mins. So drawing 9.0-9.4 or so kwh
The absolute minimum time needed to add 11kwh at 5.5kw is 2 hours. No ifs, no buts. And thats ignoring conversion loss!
2) repeatedly (but less often) a full discharge from 100% to 10% with no solar and similar numbers, will be more like 8.5-9kwh since you get conversion losses as well.
For 1 I can supply Octopus 30 minute data that semi supports this. Its more difficult since I have baseload as well of course in that time, and typically I use my dishwasher in that time period as well. Since its my cheapo elec period.
In summer I don't bother, but in winter DW goes on around the same time batteries charge.