the problem with using barrels is they run out so need filling up or changing to another one.
thats a trip back to the van to change it over which means lugging em about, sliding one out thats near the back of the van.
or as matt does refilling with a sub pump.
i started with a trolley and 6 tubs in the car. then got a van and alot more tubs, 425ltrs worth.
i worked out one day the time to clean a 3 bed semi, and the time to go back to van to change a tub over. then wait tll the airlocks have been pushed through if you've run the pump/system dry. walk back to the house etc.
i could clean another house in that time.
went to full 650 van mount a month or so ago, and its like night and day.
first day with van mount was best day ever. and has been ever since.