It costs alot of money to carry around water.We always just plug in to the customers.. I would rather waste a little time on site, then have the added fuel bill, and wear and tear on the vehicles from dragging water around..
Although I am going to set up a tank in one of our trucks this year for commercial gigs where there simply isnt a plug in close enough..
We carry around 50 gallons at a time, it's enough to work on 3 homes if the windows are just standard cleaning.
Always Carry 225 minimum