This company has a few different products and hence a few different methods to clean windows. Do you have any particular Karcher device in mind?

I've used the power washer many times [a different brand].
Ours can be made to give a long v shaped spray that as long the water is at an angle to the glass shifts all the muck really easily. if you go at it head on it will blow the glass out [voice of experience there]

Sorry I realised after posting the question about the different products made by Karcher.
Yes I do mean the power washer. I would hate to break the glass but if you said SBG that you have used a similar one I may well with care try it out.
Thanks for your answers.

stand a good 10ft back then move closer until it is cleaning the glass.less than 45 degrees to head on worked best for me.