Exactly what JimLuke said, estimating how much water you’ll need is really difficult. I have a 200gal tank on my trailer and pull it with MAYBE 50 gallons. My pump pulls straight from the tank, so I hook up to a water source and start to fill as Im working. Some jobs I use 100 gallons, others (BIG jobs) ill fill and drain my tank 2-5 times. The only reason I even have a tank is if im doing something and there’s no water source near by, I’ll still require to have a water source to fill, but I charge extra since I have to stop work to go and refill. It takes a while to stop work, wrap everything up, go back and wait for it to refill and set up all over again. Time is money and if I’m having to stop work because I’m out of water, then I should be compensated for that time.
I did a white plastic fence on a huge property - probably 15-20 acres and had to go back and fill up a few times. I charged them $40 each time I refilled. And that was honestly an hourly pay cut for me.