Adding more to this, for people who live in apartments and/or condos, but have street access, or if you want to sneak to a parking lot somewhere, or just looking for an easy way to use only de-ionized/distilled water (you can buy it in refillable 3 or 5 gallon jugs from Whole Foods for $0.49/gallon) ...
So the question becomes what if you want to spray off your car without a hose or you don't want / can't use a hose?
Or what if you want to use de-ionized water to avoid water spots but don't have a home de-ionizer and/or you want that on the road?
Well, if your application is mostly for the car, then there are pretty good battery-powered "power washers" ... Sun Jo makes them but I prefer Worx since they have a whole ego-system including a pretty cool portable car vacuum which uses the same batteries.
Worx has basically 2 models: the cheaper version that has the motor in the handle, and a more expensive version with a brushless portable motor that goes in the bucket. This guy review the expensive version here:
And the lower priced one is reviewed here:
My 2 cents is, you don't need (and shouldn't want!) more pressure than these portable units put out, and the portability gives you the option to bring this with you on road trips to wash off bugs, etc anywhere AND use distilled/de-ionized water to avoid water spots.
Basically fill up a 3 gallon whole foods water container with distilled water, or just use tap, and then spray off your car anywhere: hotel parking lots, walmart, side street, where ever. It's a great option to pair with rinseless wash as it gives you the option to power spray off your car first, and to do a pre-suds if you want.
Basically I use the process in the first posts above, but add this in to do a pre-rinse down to spray off bugs & dirt and since it's portable it's easy to bring with on road trips, skiing, etc.