It all depends on the sizes of houses your going to be cleaning , id be lucky if iv any left at dinner time 350ltrs, but majority of my custys are detached with connys so I don't get through as many in a day as some others . But as long as you've got a water butt at home for storage you can nip home have dinner fill up and back out again . I have my flow rate on 35
I have a 500ltr tank in van and my flow rate is 35. My system was a diy from pure freedon. When i first started i haf flow rate on 20, i was very cautious (? Spelling not strong point) didnt know how much water i would use even though had a valve tap on hose to conserve water. This is because at first didnt have garden space for a static tank in garden (only small garden n wife said no! Lol) so had to produce pure every night to top up van.
Now after sweet talking mrs, i have a static tank and i dont worry how much water i use anymore.
My rounds are compact and semidetach 3 or 4 bed houses. A mix of conservatorys as well. I would say, use between 200 -300 ltr a day. As my work is 4 wkly, even on 40 flow rate i use less water than when i ran flow rate of 20, as more water means better and quicker rinse. This is my experience anyways.
Yes there is a difference of flow rate depending which controller you have.