we have 2 large blocks of flats in eastbourne and hastings( that you now about) and 2 offices that we do in 1 day. 1 of the blocks is right on the seafront so always covered in salt
2 people working, 4 hours continuos poling
650 is just not enough for 2 people working a full day commercial, we used to run out of water when working for that

firm and that was 1000 litre system
even when working residential ive had to stop early a few times, i get about 5 hours poling with varistream turned to 8-9 setting when on my own