What is the minimum flow required for a pressure washer?
Most consumer pressure washers require a feed at typical municipal pressure and at a flow rate of more than the washer's output.  Karcher appears to be an exception.  

This video claims that their units can draw the water they need from a static source, like a drum or tank of water or a pool, using a siphon hose.  That water isn't under significant pressure (its own weight), but nothing limits the flow in that scenario.  If you feed it from a piped source, that source must be sufficient to feed it as fast as it needs it.  

I would verify the requirements for the specific model you're considering.  The specs should tell you the input requirements, or you can check with Karcher customer service for an official answer.