cool does that last you the day pal and how many houses does that get you roughly?
I would have thought that this would be a very difficult question to answer and his answer may not apply to you.
We live on the North Sea East coast and the majority of our windows that we clean are affected by salt sea air. We need to use more water washing and rinsing that we need to do when working a little inland. So we use around 450 to 500 litres a day each when working these coastal areas. When working inland we will use about 300 to 350 litres.
If you are new to wfp then you will use more, especially when doing first cleans. The suggestion is to process water into a 1000 litre IBC tank so you can pop back and top up if necessary. The trick is to start at the furthest place first and work back home. If you run out of water in the mid afternoon you shouldn't be to far away to slip home/back to base and top up.
I would then have a couple close to home to do that will take me the rest of the day to do.
You could also carry a few 25 litre plastic containers of water, but be careful of overloading. The Astra van has a payload of 600 kgs including you, fuel, etc. so carrying the bare minimum should keep you within payload limits.