Well if you are WFP, what is the difference between tap water and the water you use? (and yes, of course I know the actual difference!)
Pure water is little different to rainwater.
I've heard that on self cleaning glass you are not supposed to use soapy water or even a squeegee and applicator
The stuff does work though, it's a chemical reaction caused by the coating on the glass and sunlight.
doesn't stop the windows getting dirty of course, but soiling gets broken down, supposedly allowing the glass to be cleaned with a hose and ordinary tap water, or rain if it a conservatory roof...so say that is...
Ian