Thanks to all who made comments on my problem with green algae growing on the outside of my vinyl siding and that lower pressure 'washers' may do the job but maybe at a slower rate.
I will look into 'siding cleaning' agents and maybe try them, too. We've had a variety of experience with fungus (black, rather than green, smudgyness and dots) and cleaning that with soap/bleach by hand (not fun at all).
The warning about water being injected behind the siding is a valid concern and I'll have to think about this. I do know that it is not uncommon for people to power wash their homes at some interval and that unwanted side effect is certainly a possibility. I was present during one power wash of someone else's house and water did get injected behind roof flashing and started drippingfrom an interior room ceiling.
In the house we owned before the present, we had the exterior power-washed before selling the house AND also after the power wash I replaced some of the siding (cedar) because of unsightly pre-existing cracks and/or warps (the moisture barrier was, believe it or not, roofing felt) and I noticed no moisture residue or short-term rot (house & siding were about 20 years old) and I mention this as an FYI.
Art Sowers (email is invalid)