We always carry a full tank of water on our vans, between 400-650ltrs, but when running on most pressure washing jobs we use a bin constantly filled by clients water supply.
A typical sized water tank may give you an hour or two water supply tops, for big jobs when you're working 8 to 10 hours you got no practical choice but to use customers water supply, unless you have access to a standpipe nearby.
Cheers,
Huw