Days of Autonomy question

Our installer says he gave us 2 days of autonomy.
48V system, there are 16 Rolls s-530 batteries ( 2 strings) with a capacity of 800 Ah (at what I believe is the C-20 rate?) He says we can use 5.9 KWh per day for 2 days before the generator kicks in.
This sounds a little optimistic, but I'm trying to figure out how close his estimate is or isn't.
Reading another thread, someone suggested a very rough (I think pre-effeciency conversions) equation of 1Kwh = 21 Ah for the 48V system. That would give a total of 19Kwh for the 400 Ah (50% dod)
Figuring a 100% state of charge, and a dod down to 50%, can someone tell me what my ballpark might actually be?
Thanks

Sunny Boy
Sunny Island 5048U
18 Sharp 170w
16 Rolls Surette S-530 48V