Initially, you'll use a lot of water, but once you get confident, with a back pack you'll be able to do at least the tops of three houses with one fill, and it's a piece of cake to tow about if you use a fishing trolley sack truck.
I can clean my whole house (11 good sized windows and two doors) with about ten litres of water.
If I worked by myself, I'd definitely go van mounted and sack truck/back pack. But since I work with Wor Lass who won't go WFP it works for us to work together with me on tops with backpack and Wor Lass tradding behind.
But we've been doing this for about four-or-five years now and we're like well oiled machines. I tend to use her for the bulk of my compact work, drop her off home at around 2pm and then head out with more water and clean drive-betweens 100% WFP.
You'll be surprised at how many windows you can do with one fill, and re-filling doesn't take long at all. You'll work out where to leave barrels of water for bigger jobs; there's definitely a knack to using a backpack effectively, but I prefer it to a 50 litre system. And with a fishing sack truck it really is easy to cart about.
I bet there's 50 litre systems rotting in people's sheds/garages/back gardens all over the place. I took mine to the tip; it was just to big and heavy. But saying that, a window cleaner local to me has been using one for about five years now; but if I were you - if possible - I'd try before you buy.
You won't go wrong with having a back pack though; even if you go van mounted you'll still find regular use for it I bet.
good advise on the back pack , i started of with a back pack still got it , still gets used on some jobs , although i found it easier to just carry it, as apposed to putting on a trolley, i can get 3- 3 bed detatched houses out of 1 fill (tops and bottoms).
although i mainly use a van man mount , i would not be without it .
daz