There are cleaning products that can be used with a garden hose, and such. So PSI, as already mentioned, is dependent more on the condition of what you're washing than anything else. Be sure to wash from the bottom UP; NOT the other way around. And as you wash, rinse the previousl-cleaned-areas below often to keep them wet AND clean (from the 'junk' that you wash down.)
For me, I only use my power washer on my 'stuff', and the few times I use it, electric is fine w/me. (I do have outlets on all 4 sides of the house so that is nice for me. So if you have to run cords all over the place, then consider gas.) I don't lug it around on the job or anything. And if I did, I'd DEFINITELY get a gas powered unit. I've never had to use it over 1200 PSI. I use it on the highest setting when I'm cleaning off the driveway before I, say, re-seal it with blacktop sealer. Or when I'm cleaning off mold/mildew off of the concrete walkway on the north side of the house. IF you use a cleaner, then you only have to RINSE the solution off; NOT power-pressure it off. So be sure to not 'over-do' what ever you're cleaning.
FOr example, if you're just pressure washing the mold and mildew off of the driveway, you need some power. BUT, if you apply some water and bleach first, you only need to apply SOME pressure. You might want to consider renting a unit to see how it feels. My best to ya and hope this helps.