so I eventually found this ref:
https://amosbbatto.wordpress.com/2017/0 ... ar-panels/"Standard glass has a solar transmission rate of 83.7%, meaning that 16.3% of sunlight won’t reach the photovoltaic cells below the glass. In contrast, the patterned glass commonly used in solar panels prevents much of the reflection of light, so it has a solar transmission rate of 91.4%. The addition of an anti-reflective coating raises that rate to over 94% and can reach as high as 96.2% for the best glass such as ArtSolar. Using that kind of specialty glass, good commercial panels will get roughly 10% or more light than the DIY panel."
It's a long read, but full of good technical info. if iron free low reflective glass cuts about 8% less light than standard tempered glass, or considering I am using 18.5% efficiency cells, this would make them equivalent to about 17% efficency cells.
Given I obtained standard tempered glass second hand at $1/sheet (9x4 6"cells), vs $110 AUD for iron free ... I can live with that. I'll make 8% more panel area instead because it's cheaper to do so.
A.