Yes, you need the following:
Water butt (bigger the better) connected to a drain down pipe and full
25 L fresh water Container, type used by annoying caravaners
3 foot piece of hose pipe
10 foot piece of hose pipe
Hose pipe quick connector
2 jubilee clips correct size to clamp hose pipe
Piece of muslin cloth or dish cloth
Power washer of your choice ( karcher in my case)
Electric point nearby where you are washing your car, I use an extension reel
I guess you can see where I am going with this?
Connect the small length hose pipe to your water butt with the jubilee clip and cover the other end with the cloth and clamp using other jubilee clip (acts as a water filter taking out all the big pieces of crap.
Fill the water container with water from the water butt and take round to your car (helps build your muscles :wink

.
Put the hoze pipe quick connector to one end of the long hose pipe and connect to your pressure washer, works with karcher. Put the other end of the hose right into the bottom of the water container, worth using a rag to help keep it from popping out.
Plug in your power washer, switch on and away you go!!

takes a few seconds for the water pressure to build and isn't as powerful than when using mains but more than enough to wash your car.
Simples!
I only use about 2/3 the container which is a LOT less than using watering cans etc. all perfectly legal as well because you use grey water.
Questions?
Maybe we can make this a sticky??