A backpack full is more than enough for the back of a average house.
Yes, of course, with a Trigger or Aqua-dapter I would expect to be able to do quite a few. One customer who uses a back-pack told me that, with an Aqua-dapter, he uses approx 11 litres per semi.
So for backs only maybe 4-5 litres per house, roughly 3 backs of houses per refill of the backpack?
I was really thinking of a row of backs e.g. terraces or compact work in a street. If the van is at one end and the backpack user at the other, a walk to refill after 3 houses might take longer?
If they're individual jobs, then I expect it would be quicker.
Paul