In reply to Barrybra:
Silicon PV operates across an illumination wavelength range of about 300 nm to 1100 nm. The transmission of clear polymers is variable depending on the precise material. From the following: https://www.gsoptics.com/transmission-curves/ things look pretty flat across that wavelength range at about 90% transmission for common clear polymers.
You also get additional loss from Fresnel reflection which is tied to the refractive index of the material and other more complex issues involving angle of incidence and polarisation (glass @ n=1.5 ish is about 4% loss for normal incidence).
I would guess 15-20% efficiency loss overall? You could do a test with a multimeter and appropriate load.
Temperature may also be a factor, solar PV efficiency decreases with ambient temperature. Having a panel in a map case will increase the operating temperature.
Post edited at 13:19