This is the place to share all things related to power washing.
It could be a good idea to add or offer window cleaning services in addition to pressure washing services. Most pressure washers seem to offer roof cleaning services and maybe gutter cleaning. Everyone assumes that homeowners wash their own windows, when in reality, few seem to (myself included 
), and let the heavy rain/snow do some of their work for them. I’m willing to guess that if homeowners were offered a reasonable add-on fee to wash their windows, they probably would go for it, as many have hardly done this, if ever. Here’s an article that sparked the idea: