What I do is never (hardly) carry it full. I stick a 25 litre barrel of water on a sack truck and keep it handy and re-fill the back pack when it's needed.
If for example I'm going to clean a house with five 1st floor windows (I use it for tops only; mostly), I'd put about 10 litres into it (I calculate 2 liters per window (but only use about 1-and-a-half; at most for a normal sized window)).
Then refill for the next house; it takes seconds.
Sometimes I fill it to the brim, when faced with lots of windows in close proximity; but I never wear it while using the pole and rarely carry it with more than 12 litres in it; unless it's just to move it a few meters.
You'll find the above difficult on first cleans, because you use more water, but on subsequent cleans it's great.