100 watt panel, on windshield vs behind it.

[sternwake]

No shadows on any cell from windshield wipers or tint.
Facing due south.
100 watt bendable Sunpower panel.plugged into main mppt.controller in parallel with roof mounted solar

 On windshield. 87 watts
Same angle behind windshield. 43watts.

Repeated  3 times near noon with some high thin cirrus clouds. 
Behind windshield always halved output.

I was hoping for 2/3.the output,  half was disappointing,  but 43 watts is still more than my fridge draws.

Flexpanel needs lots of support when located behind windshield. Mine  formed a crease in between some cells with some.support top and bottom only.

Ill not recommend a panel in such a location without much more support, that can also allow panel to change size with heat.

[Roadtripp]

I’m surprised there’s a 45 watt difference. That’s a lot! 
 Interesting comparison. I’d be interested in any further observations you gather from the SunPower panels.

[sternwake]

My windshield has a lot of pitting. But, i have always noticed how Much reflections off windshield have lit up street signs when the angles align.
The anti reflective coatings on framed panels are not bs.

[GotSmart]

This is valuable information that answers questions that I have been asked. 

Thank you.

[rvpopeye]

What he ^ said....

My first big panel started out on my dash like that.
Quickly made a crude easel to set it on outside .

Never knew the output, no meters at that time + place.

[Blacktank]

on the outside it would make for a good heat shield too

[sternwake]

This panel was going to go on my roof, but I would have had to shave it down a bit and add some area to my roof and it would be very visible, prone to scratches and I just said F it and keep it as a lightweight easy to store  fragile portable.

I'd had some badly applied poor quality tint at the top of my windshield since 2001 which I just removed, which inspired me to finally test to see how it fits.

   

I have a strip of vertical grain douglas fir running lengthwise along top and bottom in this photo . Leaving it in on the dashboard for a half hour or so,  a vertical  kink formed in between some cells, between 3 and 4 left to right.  I've seen some put other brands of flex panels in their windshields and they looked horribly warped bent and sagging around the rear view mirror, and now I can see how easy that would be to achieve when completely unsupported in the middle.  I have added more structure to that doug fir to make a thin frame, but need to seal and paint it to prevent it from moisture  and heat cycling to the degree which it can now.

Behind the windshield facing south, the extra 43 watts around  noon is appreciated, and if it is safe from creasing, breaking intercell connections, getting blown over or stolen, the loss of the potential extra 44 watts is OK, acceptable, and when those ~44 watts are absolutely  needed it can be placed on windshield instead.

The 50% reduction in wattage is obviously disappointing. 
 Cleaning the  already pretty clean glass further, made zero difference in output.

Putting my regular reflectix window shade up behind it did not immediately change output, but I imagine with more time it would get hotter and lose voltage and thus some available wattage.  I thought the extra reflection of light bouncing off the windshield interior from the reflectix, would be good for perhaps 5 more watts, but my meter did not budge in the slightest when I did so.

[Blacktank]

windshield is laminate glass,side windows tempered,bet the laminate is playing a good part in uv passing through,not sure if late 80's? dodge used slightly tinted/anti-glare windshield like todays cars do

anything is a whole lot better than nothing so 50% is well worth it

[sternwake]

I'm not sure if my windshield is original or not, replaced before my ownership.
Due the the vast amounts of pits I would guess it is original. It does not have that greenish hue of the tempered side windows.

I plan on storing this panel on the ceiling over driver's head and make it easy to lower into window, but that crease inbetween cells bothers me. I think these 'flexible' panels should be considered bendable, yet fragile. They are certainly not designed to be flexed often in the same place. The natural bend they take when used as a portable, is opposite the bend forced into them when in the windshield, and mine did not seem to appreciate it.

[regis101]

Look for the manufacturers stamp on the windshield.  It may not be the original but can still be a factory reinstall.
However most windshield replacements are from outside sources.