Why solar panels do not work behind glass

Context of tentative PVs installation behind glass

I deciced to turn my apartment's balcony into a source of electricity, especially it is ideally oriented towards the South-West, on a hill facing the Mediterranean sea. 180W solar panels were found to fit in the available space, as per the below picture. I temporarly wired two panels in series underneath the railing and behind the glass.

On a very sunny day and against a maximum theoretical current of 9.52A (Imp), the measurements I conducted simply proved that there's no use to install PVs behind glass.

The below figures do show the Amps readings at the same moment of the day, with the solar panels installed behind the glass (left) and outside the glass (right), and indeed in the exact same place and orientation:

The conclusion leads to no confusion, taking into account the below factors:

1. Trial carried out in November
2. Location nearby the sea (high relative humidity)​
3. Location is in the city's suburbs with some air pollution 
4. Panels installed vertically ​
5. Voltage is imposed by the 2 tubular batteries in series, mounted in a floating mode on a solar controller and a battery inverter.

The solar panel generated a maximum of 6A instead of 9.52A, that is a loss of -37% compared to Imp.

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When installed behind glass, the solar panel's output peaked at 3.4A, that is a further -43% reduction or -64% from the rated maximum current.

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Installing solar PVs on a balcony

Taking the above into account, I had then to modify the glazed aluminum frames in order to replace parts of the glass with PVs...until transparent PVs are made availble in the market and can be used for windows!