I've not used one, but I have my doubts about its effectiveness.
Some cordless stuff can be OK (screwdrivers, for example).
But something like a pressure washer really needs more "oomph" (technical term!) than a battery can really provide.
Or the battery can provide the oomph, but only for a couple of minutes before it becomes little more than a fancy nozzle on the end of a garden hose.
I've seen the adverts on TV for these and in all the demonstrations - the "dirt" has clearly been sprayed or brushed on purely to stage it and such that it would just wash off even with a watering can.
They never show it cleaning any real ingrained dirt - Why?
Happy to be corrected if anyone has any hands on experience with one. But based on my experience and anecdotal conversations with friends & family with other "cordless" tools - I wouldn't risk my money on it.
And I'd be reluctant to give it as a gift - If it does turn out to be useless, I wouldn't want the recipient to feel pressured (sorry!) to use it just because it was a gift.